Fluorescence photobleaching recovery methods reveal marked changes in lateral mobilities of rabbit lymphocyte membrane components during the course of stimulation with succinyl concanavalin A (S Con A). The diffusion constant of S Con A receptors on T lymphocytes falls from 1.6×10-10 cm2/sec to 6.5×10-11 cm2/sec within 4 hr after stimulation, remains constant for 14 hr, and returns to its former value. The mobility of B cell receptors similarly falls from 1.4×10-10 cm2/sec to 5.5×10-11 cm2/sec but regains its unstimulated value much more slowly. In contrast, a fluorescent phospholipid analog shows constant mobilities of 1.9×10-8 cm2/sec and 1.5×10-8 cm2/sec in T and B cells, respectively, throughout the experiment. © 1979.