Concanavalin A (Con A) was found to induce the appearance of time-dependent and saturable thymidine transport in human peripheral lymphocytes with a Km that ranged from 200 to 500 microM and a Vmax of between 5 to 8 pmols/cell/sec; transport of thymidine could not be demonstrated in resting cultures. Colchicine was found to cause the disappearance of microtubular networks in the majority of cells in the population, whereas lumicolchicine as expected had no significant effect on the presence of microtubules. Lumicolchicine caused 50% inhibition of thymidine transport and of DNA synthesis at a concentration of about 10(-4) M; the effect of this agent on the transport of thymidine could account for its ability to inhibit DNA synthesis. Colchicine also inhibited the transport of thymidine by 50% at about 10(-4) M. However, half-maximal DNA synthesis occurred at a colchicine concentration of 5.0 X 10(-8) M; thus, the effect of this drug on transport cannot account for its effect on DNA synthesis, since the latter process is some 3 orders of magnitude more sensitive than the former. The ability of colchicine to inhibit DNA synthesis in Con A-stimulated human lymphocytes would appear to be an indirect consequence of disassembly of microtubules.