Treatment of human monocytes for 24-48 h with the anti-CD43 mAb L10 caused five- to sevenfold stimulation of hydrogen peroxide-producing capacity, an established characteristic of activated monocyts. Peroxide-producing capacity induced by L10 antibody (1.6 ± 0.3 nmol H2O2/μg DNA/h) was comparable with that induced by IFN-γ (1.3 ± 0.4 H2O2/μg DNA/h), but appeared more rapidly (maximal at 24 h) than in the IFN-γ-treated monocytes (maximal at 48 h). Treatment of monocytes with L10 mAb also caused dramatic increase in aggregation (homotypic adhesion). Induction of monocyte aggregation by L10 mAb required incubation for 1-8 h in the presence of Mg2+ and was abrogated by TA-1, and anti-LFA-1-α mAb. Thus, L10-induced monocyte activation proceeds via a Mg2+ requiring aggregation stage involving LFA-1. Whereas the extent of monocyte aggregation induced by L10 mAb and by IFN-γ were comparable, the L10-induced aggregation occurred more rapidly (maximal at 8 h) than the IFN-γ induced aggregation (maximal at 24 h). The more rapid appearance of aggregation and increased hydrogen peroxide capacity in L10-treated monocytes suggests that the L10-induced activation pathway is independent of IFN-γ and IFN-γ-R dependent events. These findings suggest that the surface molecule CD43 is the receptor of an independent activation pathway that leads in lymphocytes to proliferation and in monocytes to activation.