We have investigated the requirements of mitogenic response of mouse spleen cells for free and cell-bound concanavalin A (Con A) and inhibition of the response by egg lecithin. We also studied 125I-Con A binding with the mouse cells at different times after stimulation with the lectin. Our results suggested that mitogenic response to Con A by mouse spleen cells may be regulated by two receptor-mediated mechanisms acting in concert. Mitogenic response was inhibited during the first 2 hr following stimulation with Con A by removing free Con A from culture medium. But after 4 hr, the response became independent of free Con A and, concomitantly, Con A binding by mouse cells increased by about 2-3 fold. It was further shown that the increase in Con A binding depends on prior reaction of the cells with the lectin but it does not, on de novo protein synthesis. Treatment of mouse cells with glutaraldehyde may also increase Con A binding. These results suggested that the initial Con A binding with mouse cells may not be sufficient to initiate mitogenic response. It may however mobilize those Con A receptors which have remained hitherto cryptic but occupancy of which by Con A may be essential for triggering of mitogenic response. The second mechanism affects mitogenic response at a time after receptor mobilization and before cells are committed to proliferation. The nature of this mechanism remains obscure but it requires Con A binding and is sensitive to inhibition by the membrane active substance, egg lecithin.