Histamine releasing factor (HRF) generated in vitro by spleen cells of immunized mice activates homologous peritoneal mast cells (both normal and sensitized) for small but consistent, dose-dependent histamine and 5-hydroxytryptamine release. Both the time-course of this release and its susceptibility to the enhancing effect of D2O were similar to those observed in IgE-induced release; however, simultaneous challenge of sensitized mast cells with HRF and specific antigen or anti-IgE showed an additive effect, suggesting that mediator release by HRF and by the IgE system are independent of one another.