The role of microtubules in mitogen-induced human lymphocyte activation and proliferation was examined. The effect of colchicine, a microtubule-disrupting agent, was compared with taxol, a microtubule-stabilizing drug, and with isaxonine (N-isopropyl-amino-2-pyrimidine orthophosphate), a proposed microtubular-active drug. Lymphocyte proliferation, assessed by measuring the increase in the number of cells in mitogen-stimulated cultures, was completely suppressed by both colchicine and taxol (100 nM) whereas significant inhibition by isaxonine required much higher concentrations (5 mM). In order to characterize the inhibition, initial lymphocyte blast transformation and subsequent DNA synthesis were investigated. Neither colchicine nor taxol inhibited lymphocyte blast transformation assessed by quantitating the change in volume of the stimulated cells after a 24-hour incubation. In contrast, isaxonine (2-5 mM) suppressed blast transformation. Initial DNA synthesis, evaluated by measuring the cumulative incorporation of [3H]thymidine between 30 and 48 hours of culture, was inhibited in a concentration-dependent manner by both isaxonine and colchicine but not by taxol. Electron microscopic studies confirmed that both taxol and colchicine (10 nM) arrested the responding lymphocytes in mitosis, and that isaxonine inhibited initial activation. These results suggest that normal microtubule function is only necessary for cell division and that drug effects on blast transformation and initial DNA synthesis are unrelated to microtubules.