In a cross-over design, three interventions were tested for their impact on the rate of use of 12 commonly ordered blood tests and roentgenograms among internists in a health maintenance organization. Overall use fell by 14.2% in a 16-week period during which physicians received confidential feedback on their individual rates of use compared with peers (cost feedback). Eleven of 12 tests showed some decrease. Similar feedback on rates of abnormal test results (yield feedback) and a program of test-specific education failed to show a consistent effect. Variability in rates of test use among physicians, as measured by the coefficient of variation, fell by 8.3% with cost feedback, by 1.3% with yield feedback, and by 2.3% with education, but these changes were inconsistent across tests.