Use of physician services under two prepaid plans offered to Stanford University staff is analyzed and compared. One is a Kaiser plan; under the other (Clinic plan), physician and outpatient ancillary services are provided by a predominantly fee-for-service group practice and hospital services are covered by a Blue Cross policy. The two plans provide much the same benefits but, in addition to the difference in their organization, they differ in their financial provisions. While the Kaiser plan has only a token copayment for office and home visits, the Clinic plan has a 25 per cent coinsurance provision applying to all physician and outpatient ancillary services. Despite these differences, the mean number of physician visits per year is the same for the two groups after account is taken of differences in age composition, socioeconomic status, health status, attitudes toward seeking care, length of plan membership, family size and satisfaction with the plan. However, when adjustment is also made for differences in physician affiliation, the Kaiser rate becomes half a visit higher than the Clinic rate. This is because under both plans, members who have a specific plan physician as regular source of care use more services than those without one, and because only 42 per cent of Kaiser members compared with 87 per cent of Clinic members stated that they had a specific plan physician.