The immunosuppressant rapamycin, an immunophilin-binding antibiotic, has been studied in follicular B-cell lymphoma lines that express the highest level of the BCL-2 protein. The growth rate of human follicular B-cell lymphoma lines was slowed more efficiently than that of other human B-cell lines or non-B-cell lines. This effect was dependent on the arrest of cells in the G(1) phase; the number of apoptotic cells was not increased. Rapamycin inhibited apoptosis or caspase activation induced by cytotoxic drugs, whereas caspase activation by doxorubicin was not inhibited. The increase in the cellular concentration of BCL-2 protein was related to its concentration in the steady state and was unrelated to the amount of bcl-2 mRNA. The increase of BCL-2 level in the cells rather than its level in the steady state may be important for drug resistance. The biochemical target of rapamycin, the mTOR kinase, may be a candidate sensitising agent for chemotherapy. This effect of rapamycin shows that G(1) arrest and protection from apoptosis are combined events susceptible to regulation by pharmacological means.