William L. Roper, a physician and special assistant to the president for health policy, is the likely nominee for administrator of the Health Care Financing Administration, the agency in the Department of Health and Human Services that runs the giant Medicare and Medicaid programs.

Medicare, at an estimated $75 billion in fiscal 1986, is the government's second largest domestic program, trailing Social Security. Medicaid, with grants to the states of about $25 billion, is the largest federal-state grant program.

Former administrator Carolyne K. Davis, who had served since 1981, resigned last summer, and C. McClain (Mac) Haddow, department chief of staff, has been acting head of HCFA since then.

Meanwhile, Dorcas Hardy, assistant secretary of HHS for human development services, has reemerged as a leading candidate for nomination as commissioner of the Social Security Administration, although Wayne Stanton, HHS's Midwest regional administrator, is a possibility. Hardy was a California health department official when Ronald Reagan was governor there. Social Security, at more than $200 billion, is by far the government's largest domestic program; and the SSA has 78,000 employes, two-thirds of all employes in HHS. SSA also administers the Supplemental Security Income, Low-Income Energy, and Aid to Families with Dependent Children programs, which account for over $20 billion. Martha A. McSteen, a career civil servant who was regional SSA administrator for the Southwest, is acting commissioner.

The new HHS secretary, Otis R. Bowen, has promised to fill other top HHS jobs, including assistant secretary for health, assistant secretary for legislation and assistant secretary for planning and evaluation. Interviewed for the P&E job was Paul Ginsberg, former Congressional Budget Office expert on health economics.

Bowen last week announced the appointment of Dr. Herbert Nickens as the first director of the new Office on Minority Health. Nickens, a psychiatrist at the National Institutes of Health, served on the HHS task force that completed a study of health problems among racial minorities.