Broncho-Vaxom (BV) inhibited in dose-dependent manner the release of histamine from and degranulation of isolated rat peritoneal mast cells stimulated with compound 48/80 and the ionophore A23187. Inhibition persisted after removal of BV from the incubation medium before stimulation, but did not occur when bovine serum albumin (BSA) was used instead of BV. Binding of BV to mast cells was observed by electron microscopy on cells that had been incubated with colloidal-gold labelled BV. There was no significant difference between the binding of BV gold and BSA gold to the mast cells. Washing before fixation removed most of the BV gold from the cells. This study establishes BV as an in vitro histamine release inhibitor.