Lectins are powerful stimulants of quiescent peripheral blood lymphocytes. They can induce blast transformation leading to mitosis of these cells in vitro. We report here the dose-dependent proliferative curve for human peripheral blood monouclear cells (PBMC) stimulated by the lectin amansin, from Amansia multifida. Amansin stimulated proliferation of (PBMC) at relatively low concentrations (3.12 to 12.5 μg mL-1). We observed also a gradual reduction in mitogenic capacity with progressive increase in the lectin concentration above 12.5 μg mL-1. This decrease in the mitogenic potential did not result from a toxic effect on the cells, and was predominant at a lectin concentration above 50 μg mL-1. This decrease in lymphocyte proliferation could be blocked by avidin and could not be overcome by IL-2 or another lectin (Con Br) at stimulatory concentrations. Additionally, we observed that cells incubated at stimulatory concentrations of amansin produced IFN-γ. Analysis of the culture supernatants established a direct correlation between the IFN-γ and the mitogenic and anti-mitogenic capacity of amansin.