At 6 foot 5, John Cleese is the tallest Python in captivity. That's Python as in Monty Python--the popular British comedy troupe whose brand of humor has given new flavor to bad taste. Quick-witted skits about dead parrots, cannibalism, transvestite lumberjacks and other lighthearted topics transformed the group of ex-Cambridge students into BBC television stars virtually overnight. The half-hour shows immediately attracted a cult following when they made their way onto public television here.

The Python films, successively parodying the Arthurian Legend in "Monty Python and the Holy Grail;" the life of Christ in "Monty Python's Life of Brian;" and the meaning of life in "Monty Python's The Meaning of Life," have grossed millions--in both meanings of the phrase.

But Cleese's bread-and-butter isn't the glitter of show business--it's the business of business.

Cleese is one of the most prolific, entertaining and best-paid writer/actors of radio and television commercials in the world. In fact, he's quite literally his own multinational advertising agency.

In February, American Express departed from its "Do You Know Me?" television campaign to introduce Cleese as that snooty British fellow in the stuffed shirt whose butler qualified for an American Express card while he didn't.

"He's done mayonnaise for Norway, fish fingers for Australia, Asiaweek (magazine) in Hong Kong, promotions for the Danish Shoe Council and Lowney's Peanut Butter Cups in Canada," says David Wilkinson, Cleese's agent.

Cleese's varied advertising accounts have been quite lucrative for him, accounting for more than a third of his income. But the real cash cow for Cleese are his industrial training films. Cleese has written or starred in 35 such films, each one featuring Cleese as a woefully errant businessman in search of competence.

Cleese has also won considerable critical acclaim for writing, acting and producing his own BBC television series--Fawlty Towers. There he plays hotel proprietor Basil Fawlty--described by one reviewer as "the nastiest hotelier since that innkeeper in Bethlehem."

"I've always been able to pass myself off as a professional, reliable chap who, due to a bum steer, ended up in show business," says Cleese. It's always been part of my strength to come on respectable and then go quietly mad--or rather, noisily mad."

That's very much the key to Cleese's appeal: the anticipation that this enormous, tautly wound rubber band will snap. Watching someone who looks to be the Upper Class Incarnate skid into lunacy is simply great fun. Besides, people find it impossible to resist the sort of person who would list "gluttony and sloth" as his leisure time pursuits in Who's Who.

He graciously but firmly declines to disclose his total earnings. "You Americans really like to know those sorts of things, don't you?" he says.

But an informed guess would put the sum close to $300,000 a year--although the near-punitive rates of the British tax structure probably leaves him with considerably less.

Although Cleese likes to say, "Money has never been of the slightest concern to me--all I'm interested in is 'Do I have enough?' "--he is really obsessed with the stuff like a Jack Benny with a BBC accent.

"I've spent all these years making myself secure financially," Cleese says slowly, "I seem to be damn determined to have a totally secure financial base."

It was precisely that concern that led Cleese to use business as a hedge against the foibles of show business. British television actors are merely adequately paid contract employes. Indeed, Cleese's stint as the progenitor of Fawlty Towers--a full series of TV shows for the BBC--made him somewhat less than $60,000. There are gaffers in America who make more than that.

That sort of life generates insecurity for someone with a lower middle class background who attended the finest of British public schools and read law at Cambridge. "My father liked to say that there was a tradition of law in our family," says Cleese. "What it meant was that my grandfather had been a solicitor's clerk."

Instead of going into law and looking respectable, Cleese decided to become a comedy writer and performer. He first appeared on British television when he was 24 years old in 1966 on a program with entertainer David Frost.

A couple of years later, Cleese and a few of his Cambridge pals teamed up to launch Monty Python--a show of unrelenting silliness and acid humor. When sketches would come to a comedic cul-de-sac, the actors would simply turn to the camera and intone, " . . . and now for something completely different."

With his great height and patrician bearing, Cleese could play the soggiest of Tory wets or an inspired lunatic with the physical presence of a deranged stork. Python became a phenomenon and Cleese's multiple roles, including those as the Permanent Undersecretary in the Ministry of Silly Walks and the Upper Class Twit of the Year, became comedy classics. But the money and security Cleese wanted simply wasn't there.

So it was advertising, Cleese and his agent figured, where his considerable talents could be parlayed into pounds.

"I've always had a vulgar commercial streak in me," says Cleese, "and I discovered that the scale of remuneration in advertising was quite insane. When I realized it was insane, I decided I would accept it as it was. I decided I would get my foot in the door and I began doing voice-overs."

A few years later, the Sony Corp. gave Cleese an exclusive radio contract for a staggering amount of money. "When my agent told me what Sony offered," Cleese recalls, "I rolled off the couch laughing. The sum was very useful, though."

For Sony, Cleese established the style of speaking to the audience "as a silly-assed, slightly pompous Englishman . . . but being very upfront about doing a commercial. You know, 'I'm frightfully sorry, this is another Sony commercial . . . ' By laying it on the line, you've broken a barrier with the audience. The campaign won all the radio awards going."

For the Danish Shoe Council, Cleese retraced his steps at the Ministry of Silly Walks, stretching and twisting his long legs into pretzelesque poses.

Representing Callard & Bowser, the British toffee company, Cleese made advertising history by producing the first American radio commercial that didn't mention the sponsor's name. (Cleese says that British firm discovered that it had to yank the ad off the air because of a Federal Communications Commission rule stating that commercial sponsors must be identified.)

Cleese's stuffed shirt portrayal for American Express fit perfectly the image American audiences are looking for, he says. "Americans have always gone for the slightly stereotyped Britisher--like Robert Morley," says Cleese, "slightly refined, slightly superior and always holding something back."

"I only understand certain types of audiences," says Cleese, class self-consciously, "I wouldn't know how to go for the mass market. I advertise insurance, or watches or credit cards . . . things that are upmarket."

The same stuffed-shirtiness turned out to be perfectly suited for training films. Around the time he was moving into commercials, Cleese was lured into the film work by Anthony Jay, a friend of his from David Frost days.

"I had encountered this world of training films," says Jay, "and discovered two things: One was that the films were very bad; and the second, was that they were making a lot of money. It was the closest you could come to a profitable film industry here in Britain."

Jay convinced Cleese that ownership was better than mere employment and that, if the films were made right, they would be perennial sellers. "We put our savings together," says Jay, "made our first film for 4,000 pounds, and have always operated without debt and solely from cash flow since then."

"We're now running at $6 million a year," says Jay, "and we're reckoned to be the world's leading producer of training films."

"John's rewards from these films will be a durability over the years to come that he couldn't have from the Pythons. These films will be doing well 10 years from now. They really are a hidden treasure trove of John Cleese."

Indeed, Visual Arts's best seller "Meetings, Bloody Meetings" is a droll bit of work with Cleese as a manager who botches meeting after meeting through inept preparation and confused intent. It is far, far funnier than the typical training film. "Most of them are really rather solemn," says Jay, "There's no sparkle in the performance. John takes care of that for us."

An official of Xicom, Visual Arts's American distributor, says, "The films have done very well for us--Meetings, Bloody Meetings is our best-selling title." The company is planning a marketing push for the fall.

But despite the success, Cleese now feels it's time for a change. "People start climbing the ladder and they find the ladder is rather chancy. Usually, somewhere about 35 or 36, people start glancing around and wondering whether they're on the right ladder. For too long, I've been doing a certain kind of thing for no other reason for doing it than I was successful at it. I have no idea where I'll be in three or four years."

Cleese says he'll probably read a lot and write a screenplay or two--but his career as a Python is over. However, his Visual Arts works may have inspired him to become more of a media mogul and produce a few small features of his own.

"It might be fun to find out how the wonderful world of film financing worked," Cleese remarks, "It fascinates me that any business can be run so badly."