The defeat of a prominent Republican moderate in the first primary of the year produced conflicting interpretations yesterday, but the conservative victor said it would be a mistake to regard it as a signal for the GOP to turn further right.

State Sen. Kenneth G. McMillan, who defeated eight-term Rep. Tom Railsback, by about l,000 votes in the most significant contest in Illinois' primary Tuesday, said, "My personal view is that I don't think the big issue was a referendum on the Reagan administration or a turn toward more conservative politics."

Instead, he said, "a unique combination of things" weakened Railsback, including redistricting, farm-group opposition and resentment of the tax break members of Congress voted themselves last year.

But McMillan also had criticized the veteran Judiciary Committee member, a prominent figure in the impeachment proceedings against President Nixon, for having the lowest Reagan support score in the Illinois congressional delegation.

The National Conservative Political Action Committee, which gave McMillan $2,000 in cash and services, said his win "proves that even the most senior incumbents are vulnerable if they don't listen to the conservative voices of their constituents."

But Railsback said that despite the help McMillan got from NCPAC, antiabortion groups and the National Rifle Association, "a more important" factor was McMillan's aid from the Illinois Farm Bureau in the redrawn, more rural district. McMillan had worked for that organization and for former secretary of agriculture Earl Butz.

Railsback was one of four incumbents squeezed out of Congress by redistricting and the primary. A fifth's fate hung on disputed and incomplete vote counting in Chicago.

The other victims of the day were:

Rep. Robert McClory (R), ranking minority member of Judiciary, who yielded his district to Rep. John Edward Porter (R) without a fight, when the Illinois delegation was reduced from 24 to 22 seats.

Twelve-term Rep. Edward J. Derwinski (R), a senior member of both Foreign Affairs and Post Office and Civil Service committees, lost narrowly to his friend, Rep. George M. O'Brien (R), in their combined district, most of it old O'Brien territory.

Rep. John G. Fary (D), a stalwart of the Chicago organization who was rewarded with a House seat in 1975 by the late mayor Richard J. Daley, lost it decisively to Alderman William O. Lipinski (D), an ally of the mayor's son, Cook County State's Attorney Richard M. Daley (D). Fary's backdoor support from Chicago Mayor Jane Byrne (D) could not overcome Lipinski's edge in the Daley base on the southwest side.

Finally, freshman Rep. Gus Savage (D), criticized for the worst voting attendance record in the delegation, was a few hundred votes ahead of former Chicago Transit Authority chairman Eugene M. Barnes (D), Byrne's choice. But neither Barnes nor state Rep. Monica Stewart (D), a third candidate, conceded, pending completion of the count and resolution of widespread charges of voting irregularities.

Except for Railsback's Rock Island County base, where the heaviest GOP primary vote in history almost enabled him to overcome McMillan's rural lead, voting averaged 25 percent of registered voters.

Former senator Adlai E. Stevenson III (D) had no opposition for his party's nomination for governor, and Gov. James R. Thompson (R) took 82 percent of the vote against two relatively unknown challengers in the first step of his bid to become the state's first third-term governor.

Thompson's choice, state House Speaker George Ryan, won a plurality victory over state Rep. Susan Catania (R), a liberal maverick and strong supporter of the Equal Rights Amendment, and state Sen. Donald Totten (R), the leader of the Reagan efforts in Illinois in 1976 and 1980.

In victory, Ryan threw salt on the wounds of ERA backers, who had mobilized nationally to help Catania, saying, "I always thought ERA was dead. I'm sure it's dead now."