Pam is the 30-year-old office manager in a small machine-parts distributing firm. The salary isn't all that great, but there's a fringe benefit she has come to count on: Her Christmas bonus.

In good times, she can expect an extra month's pay in her check. In leaner years, the minimum has been $1,000. A tidy sum, either way, and she realizes it.

"I've already spent most of it," she says, a large part for Christmas presents. The rest will cover bills that have mounted up in anticipation of her boss's customary year-end gift.

Says a former insurance claims clerk: "Speculation began three months in advance about how much it was going to be." (Usually $500 to $1,000.) "Our coffee breaks were consumed by the discussion."

Joy to the world, if the boss is generous.

In this season of good cheer, many business firms across the country take a cue from Santa. Depending on company policy, gifts could be substantial cash payments to a few, fruit delivered to everyone's door, or a variety of possibilities in between.

The 2,684 employes of Lincoln Electric Co. of Cleveland, manufacturer of arc-welding products, will enjoy a particularly cheery Christmas this year. The firm handed out $59 million (a company record) in incentive bonuses last week, based on the year's profits. The average employe, says the firm's Richard Sabo, received the equivalent of an entire year's pay. (The amount varied according to pay scale, merit rating and length of time employed.)

Closer to home, Behnke Nurseries of Beltsville has what general manager Alfred Millard calls "a very liberal bonus policy." This year, it will divide $50,000 among 60 to 70 employes, based on "seniority and merit."

University Research Corporation of Bethesda, a consulting firm with 135 staffers, will ship grapefruit and oranges to everyone's home. Fruit, explains personnel director Ros Levy, "because it doesn't get the vegetarians upset. Everybody gets the same thing. No giant package for the president and a single grapefruit for the guy in the Xerox room."

Raleighs clothing stores adds "a token gift" to each paycheck, says the personnel department's Jean Doing, "based on years of service."

At Wilson Pontiac in Silver Spring, mechanics get a bonus based on the number of hours they worked. For the auto sales force, it is the number of cars sold that counts. Says office manager Ray Hunter: "If business is good, the bonus is better." (Hopes, he concedes, aren't that high in the showroom this December.)

To find out how widespread the Christmas bonus is, Prentice-Hall publishers annually surveys industrial plants, offices, banks and hospitals. Of 203 firms responding: "More than half plan to give their employes either gifts or bonuses" or a combination this month. Of those who do, 52 percent will make it cash, 38 will offer gifts--traditionally turkeys, hams, fruit, pecans--and 9 percent, a combination.

Among those sampled:

* Wisconsin bank: $10 to each employe.

* North Carolina clothing maker: $50 plus $2 for each year on the job.

* Colorado manufacturer, up to 5 percent of gross annual salary based on length of service.

* Missouri manufacturer, between one and four weeks' pay depending on length of employment.

* California research group, two weeks off with pay.

On the other hand, Douglas M. McCabe, professor of labor relations at Georgetown University, cites a new Bureau of National Affairs survey of 250 companies in which fewer than 20 percent say they will be giving bonuses or gifts.

Many, the survey reports, have begun offering certificates or merchandise catalogs so employes can choose their own gifts, usually in the $10-$25 range. Manufacturing industries appear to be the most frequent gift-givers, with firms having fewer than 1,000 employes also more likely than larger companies to fatten the Christmas check.

Two other surveys suggest that 20 to 30 percent of American firms give a Christmas bonus. In 1979, the Conference Board, a New York business research organization, found 23 percent of 354 companies consulted had a bonus plan. In 1980, Dartnell, the Chicago business publishers, reported 28 percent of 300 firms contacted spread some form of holiday cheer.

In many larger companies bonuses tend to go only to top executives -- those who have the most "impact" on profits -- says William Jaffe, vice president in the Washington office of the management-consulting firm of Towers Perrin Forster and Crosby. To give everyone a bonus, even a small one, is often seen as unrealistically costly.

And "Sometimes management is accused of being chintzy," says Jaffe. "There's a feeling in this day and age that $25 or $50 doesn't mean a hell of a lot."

If your boss ignores the spirit of giving? Don't automatically conclude you're working for a Scrooge; not everyone in the business community agrees that the Christmas bonus is a good thing.

Levy, for example, recalls working in a law firm where cash bonuses of varying amounts were handed out in December. As the carols were proclaiming "Peace on Earth," staff members were "worrying about how much each other got."

That's why she favors the fruit package: "If you're trying to create a good feeling at the end of the year, it's much more reasonable."

If Levy sees the bonus as a simple year-end "thank-you," social psychologist Jerald Jellison of the University of Southern California views the gesture more cynically, as a possible prod to keep workers toiling.

Year-end merit bonuses, says Jellison, are presented frequently "as a reward for a job well done," but the boss may be using the bonus as a dangling carrot. ("If you want that bonus again, you'll have to do more and more work.")

To get a $500 or $1,000 bonus, "you may have to put in 80 or 90 hours a week.

"Some people dangle a bonus for six to eight months," he says, but then don't come up with it, apologizing, instead, for "a negative cash position" or "sudden market reversals."

Jellison also urges employes "to pay attention to what you might be giving up" to get that holiday bonus. Such as a pay raise.

The bonus, he says, is a way "to give a person $1,000 now versus a $1,000 raise for 20 years--including fringe benefits. There's a huge difference. It's going to cost you a lot."

If you should be handed a check, Jellison suggests first off looking happy about it and then saying something like, "I'm really pleased to get this bonus. I assume it means I'm doing a good job. And since I'm doing such a good job, can I also expect a salary adjustment?"

Jellison insists he's not opposed to cash credits; "I'm all in favor of it, but I'm trying to warn people to be wary."

Some other management ploys he's spotted as a consultant specializing in "the power element" in business relationships:

* "The boss tells you to keep the amount of the bonus private -- 'just between you and me.' You don't tell for fear he'll retaliate."

* The boss acts "as if it is special money -- 'I've dipped in my pocket.' But the amount has been budgeted all along. Employers give the impression of acting out of altruism, but they usually expect something in return."

* "You knock yourself out all year, but then tell the boss you're leaving" in January. The result: no bonus. "They're not giving them out for what you did. They're paying to keep you producing."

As a defense against such tactics, he suggests the employe "get as much information as you can about the criteria for bonuses. This might prove difficult, since ambiguity works to management's advantage. But try to get the company to spell out these standards in objective terms."

If one of Jellison's purposes is to alert employes, he's got some advice for managers, too:

Companies which give the same bonuses to everyone, he says, "are missing out on an opportunity. If salary is relatively fixed, the bonus represents the only discretionary power an employer has, the only way to increase productivity.

"If you just give people money and not make it contingent on productivity, it becomes taken for granted."

Employes who get a merit bonus one year and not the next, he says, tend "to get all sorts of fantasies up." As the overlooked staffer, "You don't have any information. You think they don't appreciate you, they don't like you. You fluctuate between anger and fear."

To avoid this situation, in the case of his own staff, "I don't lead anybody to expect a bonus." When he does give a bonus, as he did this season to two workers for extra efforts, "I let them know clearly that it's a one-time thing."

Motivation specialist Edwin A. Locke, a University of Maryland professor both of business and management and of psychology, agrees that annual bonuses given to everyone "won't motivate performance. They become a part of the fringe benefits to meet the competition."

In principle, he believes merit bonuses are "a good idea. The problem is getting them applied based on merit. You need an objective performance appraisal or people may perceive them as an inequity." He suggests merit bonuses be awarded throughout the year rather than just at Christmas.

Whether they are a good idea or not, few people would dispute the cheer of holiday green -- negotiable variety -- at this time of year. 'Tis the season, as they say, to be jolly.