Ro 5-4864 (4-chlorodiazepam), PK 11195 (1-(2-chlorophenyl)-1,3-dihydro-1-methyl-propyl)isoquinoline carboxamide) and diazepam inhibit, in a concentration-dependent way, the proliferation of V79 Chinese hamster lung cells (IC50 values were: 65.0 +/- 3.73 microM, 57.70 +/- 4.75 microM and 106.80 +/- 8.89 microM, respectively) without being cytotoxic. This antiproliferative effect is mediated by mitosis arrest in the G2 + M stage without affecting DNA synthesis and seems unrelated to a specific interaction of these drugs with the peripheral-type benzodiazepine receptor.