The Washington Post today takes a large step into a new printing era. The newspaper you are reading is the first edition of The Post entirely printed by computerized photocomposition in place of the Linotype machines that since the late 19th century have been the keystone of newspaper production.

The Post is one of the last large American newspapers to convert to photocomposition, a process that Publisher Donald Graham noted greatly increases the speed of typesetting and will make possible more and later news in each day's paper.

With today's edition, The Post completes a process begun nine years ago. The front section of The Post today joins the other parts of the paper in having been produced with streams of electrons, beams of light and a computer's memory replacing typewriters, paper and molten lead.

In the new process, a reporter sits down not at a typewriter, but at the red, white and blue keyboard of a computer terminal attached by an electric cable to a video screen that hums gently thanks to a small fan that keeps it cool.

The hum and the muted tapping of plastic computer keys have replaced the clacking of typewriters, but to green letters shining on black video screens.

In addition to a typewriter keyboard, the computer terminal has more than 60 other keys for editing and then transforming the screen's green letters into black type.

The main news sections are using a system designed by the Raytheon Co. to the Post's specifications under a $7 million contract. "We believe it's the best there is," said Don Rice, the Post vice president of operations. Other sections of the paper are using a different technology but will switch to the Raytheon system before the end of the year.

The Post's system has 292 terminals, including 15 spares, which is a very large number to have attached to a single data base. Raytheon hopes to market the system to other large newspapers and is having initial discussions with the Los Angeles Times. When a reporter finishes a story, instead of carrying it to an editor's desk, he strikes a key marked "Done," and the text he has written is transferred automatically to the control of his editor. An editor can insert, delete, transpose or add words and sentences on his keyboard and then -- once he is satisfied -- send the story on to become a column of type.

The editor has keys that can command the desired type face and size and that can command the computer to hyphenate the story and arrange it so that the type is even with the left- and right-hand margins of the column.

The type is printed out as fast as 1,800 lines per minute as a beam of light scans photographic printing paper and arranges the words in black and white just as you are reading them now.

The entire process is controlled by a bank of computers with built-in redundancies so that should one element fail, another will keep the system operating. Even so, the Raytheon system was introduced gradually at the Post to shake out any early problems and give employees time to learn how to use it.

"No introduction of new, complex machinery goes perfectly from the outset. We'll have to get the bugs out of both the Raytheon system and the new Virginia plant's technology," Graham said.

A big part of the change at The Post, Graham said, is the new printing plant in Springfield, Va. It contains three huge new presses -- two made in the United States and one made in Japan -- that are the first offset presses operated by a Washington daily newspaper. They will mean dramatically improved quality of reproduction for papers printed at the new plant. Next year, the Post will switch to a plastic year plate process at its 15th Street headquarters, a move that will bring a striking improvement in quality for the rest of The Post's readers.

Graham said that The Post has been making major expansions roughly every 10 years. It moved into its headquarters at 15th and L streets NW and bought the Times-Herald in the early 1950s, added new presses in the early 1960s and moved into a new main building, adding still more presses, in the early 1970s.

The expansions have been driven by increased circulation, which was about 400,000 in 1960, 500,000 in 1970 and 600,000 this year. In addition, Graham said the appearance of the paper has been deteriorating lately because the hot metal equipment is so old it has been difficult to maintain. Some spare parts have had to be ordered from Europe because there is no longer an American source of supply.

Photocomposition is a marvel of 20th century technology, but the Linotype it replaces was no less marvelous in its time. It became the most important printing invention since Gutenberg's movable type when a young German immigrant, Ottmar Mergenthaler, unveiled his machine in Baltimore in 1883.

Without the clanking Linotype, which devoured long, lead ingots and spewed them forth at the command of a skilled operator as lines of metal type, mass circulation newspapers would have been impossible.

Not only the Linotype, but its operator, the printer, is a casualty of the photocomposition revolution. Linotype operators, in effect, copied the stories that had already been through the reporters' typewriters. That step is no longer necessary.

The International Typographical Union representing the printers argued that the printers' duties were being taken away from them and given to journalists. At paper after paper around the country in the 1970s, newspaper managements said that wasn't what was happening. The duties were being taken away, but they weren't being given to anyone else, management said, they were being eliminated. The management view prevailed, often after long and bitter confrontations.

The Post and its printers signed a contract in 1974 after 14 months of negotiations that gave the printers lifetime job guarantees in return for a number of concessions, including management's right to use photocomposition equipment.

That contract expired last year and once again, negotiations between the Post and the printers have dragged on for more than a year. William Boarman, president of Columbia Typographical Union No. 101, the printers' local, said The Post doesn't want to leave us anything to do.

There are 480 printers still working at the Post. Management has offered $20,000 to each one to buy out his lifetime guarantee in an attempt to reduce the workforce, but Boarman said other papers have offered much larger buyouts. tIn the meantime, the printers are pasting the photographically produced type onto the page forms, and also are putting into the electronic system typewritten material that come to The Post from outside.