The LAM1 molecule is a member of the new family of cellular adhesion/homing molecules that contain a lectin-like domain at their amino-terminal end followed by an epidermal growth factor-like domain and short consensus repeat units like those found in C3/C4 binding proteins. Two mAb that react with the leukocyte adhesion molecule 1 (LAM1) were produced and used to examine the cell-surface expression of LAM1. The anti-LAM1 antibodies were reactive with the majority of blood lymphocytes, NK cells, neutrophils, and monocytes. LAM1 was also expressed by subpopulations of phenotypically immature and mature thymocytes. Blood lymphocytes rapidly modulated LAM1 from the cell surface during PMA exposure for 60 min. Coordinate with the loss of LAM1 from the cell surface, PMA-treated lymphocytes lost the ability to bind to lymph node high endothelial venules, indicating that expression of LAM1 may play a role in lymphocyte homing. Mitogen stimulation of blood T and B lymphocytes also resulted in decreased LAM1 expression, but at a slower rate. LAM1 was only weakly expressed by a minority of spleen lymphocytes. However, culturing spleen lymphocytes in media alone resulted in increased expression of LAM1 by a subpopulation of the cells (40 to 60%). Concomitant mitogen stimulation of spleen lymphocytes resulted initially in down-regulation of LAM1 expression followed by increased expression of LAM1 and then subsequent loss of LAM1 from the cell surface. The pattern of anti-LAM1 antibody reactivity was identical to that reported for the TQ1 and Leu-8 antibodies, and all of these antibodies reacted with cells transfected with the LAM1 cDNA. Thus, LAM1 is broadly expressed by leukocytes, and binding of LAM1 may participate in the process of leukocyte extravasation into lymphoid organs or sites of acute inflammation with subsequent loss of LAM1 from the cell surface.