The relationship between the basal (spontaneous) motility of murine lymphocytes and their position in the cell cycle was examined in a new collagen gel motility assay system. Concanavalin A-stimulated or control lymphocytes were allowed to locomote into slabs of type I collagen gel. The assay configuration permitted extraction of both total populations and locomotory subpopulations as viable, single-cell suspensions suitable for phenotypic and cell analysis. Concanavalin A stimulation resulted in a significant increase in the mean distance traveled by the leading cell front in 4 hr, from 23 microns (controls) to 67 microns. The estimated percentage of motile cells increased from 0.9 to 2.8%. Similar increases were observed after 18 hr of locomotion. The SIg+, Thy-1+, L3T4+, and Ly-2+ subsets exhibited equivalent increases in motility. Total populations and locomotory subpopulations were allowed to incorporate 5-bromo-2'-deoxyuridine, and their cell cycle profiles were compared by dual parameter anti-5-bromo-2'-deoxyuridine, propidium iodide fluorescence analysis. Total population and locomotory subpopulations did not differ significantly with respect to the ratio G0/G1:S, indicating that lymphocytes in these two phases exhibited approximately equal motility. Cells in late S and G2 + M were significantly less motile; locomotory subpopulations contained 60 to 75% fewer G2 + M cells than the total populations from which they were derived. Taken together, the results indicate that the concanavalin A-induced increase in motility commences before S phase and that motility diminishes shortly before or during G2 + M.