Dr. Donald S. Fredrickson, director of the National Institutes of Health since 1975, will keep his job as a first step in "isolating" the big federal research center from party politics, Joseph A. Califano Jr., Secretary of Health Education and Welfare, announced yesterday.

"Politics is out at NIH," Califano told more than 600 applauding NIH workers, part of the 12,000 at the institution subjected to political pressure during the Nixon years.

But "depoliticization does not mean scientists can be isolated from human concerns," Califano warned. He said scientists must still expect pressure to improve medical care and conquer diseases and do so at lower costs.

Also, he said, NIH and the nation's universities must make top jobs available to minorities and women. "I say in a nonpolitical way that we've lost eight years on this problem. We can't make up for them all in four years, but we're going to try to make up for some of it."

Scientist at NIH and throughout the country - who share NIH research dollars - felt Califano's position on Fredrickson would signal the Carter administration's stance toward medical research.

Two successive NIH directors were fired in effect in the Nixon-Ford years, one for opposing budgets cuts, the other for what was called "poor administration." These acts, political screening of NIH advisers and other measures prompted protests from scientists who said NIH might no longer be able to attract first-rate workers.

Fredrickson's retention was urged by several leading scientists and members of Congress, including Sen. Warren G. Magnuson (D-Wash.), chairman of the Senate health appropriations subcommittee.