Nine antimicrotubule benzimidazole derivatives tested in a Pneumocystis carinii culture system with human embryonic lung fibroblast monolayers inhibited organism proliferation. The concentrations of drugs inhibitory in culture ranged from 10 to 0.1 micrograms/ml, with thiabendazole being the least effective (10 micrograms/ml) and parbendazole being the most effective (0.1 microgram/ml). The parent compound, benzimidazole, was inactive at 10 micrograms/ml. Demonstration that this group of compounds has activity against P. carinii provides a new potential target that can be exploited, the microtubules. Also, the variability in the effectiveness of the compounds provides the basis for studies of structure-activity relationships, which were initiated in this study.