The locomotor capacity of human blood lymphocytes taken directly from blood or cultured in various ways was measured by the change from a spherical to a polarized shape which occurs within minutes of adding locomotor stimulants. A minority of lymphocytes, either direct from blood or after culture in human serum albumin or fetal calf serum for up to 72 hr, responded rapidly to such stimulants, but most lymphocytes failed to show any shape change. Colchicine induced the highest proportion of polarized cells, though still below 50%, and deuterium oxide, which stabilizes microtubule assembly, inhibited shape-change, suggesting that microtubules have a regulatory function in the expression of lymphocyte locomotion. However culture in the presence of mitogens, namely, phytohaemagglutinin (PHA), PPD, mixed lymphocyte culture, or anti-T3 (OKT3 greater than or equal to 25 pg/ml), caused a majority of lymphocytes to change shape slowly over a period of hours. In the presence of mitogens, a high proportion of cells was already polarized after 24 hr in culture without addition of further locomotor stimulants. It was concluded that locomotor capacity in lymphocytes is dependent on growth and synthesis for the following reasons. (i) There was a direct relationship between size and locomotor morphology in PHA-cultured lymphocytes. Those lymphocytes that increased in size also became polarized. (ii) Autoradiography showed that the polarized cells were more active in [3H]uridine and [3H]leucine uptake than the spherical cells. This relationship was obvious in PHA-cultured cells but was also evident even in cells direct from blood. The increase in locomotor morphology preceded detectable DNA synthesis ([3H]thymidine uptake). (iii) Increase in locomotor capacity in culture was inhibited by cycloheximide but not by mitomycin c. These findings suggest that those cells most active in RNA and protein synthesis are also the most actively motile, and that, during culture with mitogens, locomotor capacity increases as G1 phase progresses and prior to the commencement of DNA synthesis.