The effect of a purified PHA-B fraction of Agaricus bisporus lectin on insulin release and 45Ca2+ uptake by islets of Langerhans was studied. It was found that at 2 mM glucose +17.5 micrograms/ml A. bisporus PHA-B (which produces half-maximal stimulation) there was a 3.3 fold increase in insulin secretion as compared to controls (2 mM glucose). Stimulation of insulin release was found to be dose-related as well as time-dependent. Maximal stimulation of insulin release was obtained at a lectin concentration of about 60 micrograms/ml and proceeded as a linear function of time for about 1 h. It was found that in the presence of 2 mM glucose +17.5 micrograms/ml lectin islet 45Ca2+ uptake was enhanced more than 11-fold as compared to controls. 45Ca2+ uptake was found to be lectin dose- and time-dependent and maximal uptake was achieved at a lectin concentration of about 60 micrograms/ml after which the rate slowed down until it reached a stationary phase at about 75 micrograms/ml.