Edward G. Jefferson is making clear he won't try to be another Irving S. Shapiro, the man whose shoes he's slated to fill May 1 as chairman of Du Pont Co.

If anything, the two are contrasts.

Shapiro, 64, the son of Lithuanian immigrants, was a free-swinging government lawyer before joining Du Pont. His mark as chairman was made outside, as a spokesman for American business.

It's been a job he's plainly relished.

Jefferson, 59, a dapper, British-born chemist, has grown up as a Du Pont Co. insider, virtually unknown either to the business community or on the Washington scene. He also has a ruddier complexion and is more reserved.

But the two men have shown so far that their differences are more than just skin deep.

Last week, Jefferson struck out on his own by unabashedly endorsing President Reagan's new economic program -- an anathema to Shapiro, who had doggedly backed former president Carter.

And the incoming Du Pont chairman seems likely to be noticeably more conservative, at least in his for-public-consumption rhetoric. Shapiro liked to play something of a business-statesman's role.

Yesterday, in Du Pont's ornate board room, Shapiro sat back as his successor -- newly named to the post by Du Pont Co. directors -- fielded questions in his first press conference and underscored his direction.

Besides endorsing Reagan's tax-reduction and spending-cut plans, Jefferson called publicly for a rollback of some existing clear-air standards -- and indicated Du Pont would fight vigorously to see them revamped.

At the same time, however, he insisted Du Pont officials would "continue to be good corporate citizens" in obeying environmental rules. He said the firm wants regulations that "are soundly drawn, but don't cost more than they should."

And he indicated he'll place more emphasis than before on certain corporate goals -- such as finding lower-priced, more reliable sources of oil for petrochemicals, and developing new lightweight plastics for airplanes and cars.

A good-humored, yet no-nonsense sort of man, Jefferson retains a trace of his British accent, which -- combined with his thinning gray hair -- gives him a faint resemblance to journalist and TV host Alistair Cooke.

Jefferson joined Du Pont in 1951 as a chemist at its Charleston, W.Va., plant, and married a Charleston resident. He became a U.S. citizen in 1957, and -- true to American practice -- he acquired a nickname: Jeff.

Holder of a doctorate from King's College, Jefferson worked first in plastics and fluorocarbons and later in explosives, polymers and film. He became a corporate director in 1973.