The objectives of the present studies were to develop sensitive, reproducible methods for detecting mutations in normal human fibroblast cells and to demonstrate dose-related mutagenesis by known and potential carcinogens. The authors have modified conventional test procedures for detecting mutations in human fibroblasts to enhance the system's sensitivity. These modifications include the use of cell synchronization, multiple treatments and exogenous metabolic activation. In bacteria and viruses, mutagens act primarily on replicating DNA and preferentially at the replication point. They increase the mutation frequency of a given gene most effectively if administered while that gene is being replicated. Experiments with synchronized mammalian cells have indicated that DNA synthesis is required for mutatation.