Abstract Bestatin, (2S, 3R)-3-amino-2-hydroxy-4-phenylbutyryl- L-leucine, is a small molecular immunomodifier. Effects of this compound on human immune function were studied, in vitro, using the human B-cell colony formation technique. B-cell colonies were obtained from enriched B-cell populations placed in conditioned methylcellulose medium containing stimulators and irradiated T-cells as feeders. Addition to the culture of Bestatin at concentrations of 0.1 μg/ml and 1 μg/ml led to a significant increase (P < 0.05) in the number of B-cell colonies and this effect was abolished when irradiated T-cells were not added to the culture. Bestatin increased soluble factor production induced by phytohemagglutinin (PHA)-stimulated T-cells. Such findings suggest that T-cells probably mediate this stimulatory effect of Bestatin on B-cell colony formation.