Marked IgE-mediated histamine release from rat mast cells sensitized in vitro with mouse antiserum occurs in the presence of added Ca++ and phosphatidylserine (PS), although a considerable degree of antigen-induced histamine release which may utilize intracellular or cell-bound calcium is also observed. The decay in the responsiveness to Ca++ of the sensitized cells stimulated by antigen in Ca++-free medium in the presence of PS is relatively slow, and maximum release is produced by Ca++ added 1 min after antigen. Histamine release also occurs when Ca++ is added after PS in the absence of antigen to the sensitized cells suspended in Ca++-free medium. Unlike the antigen-induced release, the intensity of this non-antigen-induced release varies depending on both mast-cell and antiserum pools. A heat-labile factor(s), which is different from antigen-specific IgE antibody and is also contained in normal mouse serum, is involved in this reaction. In the antigen-nondependent (PS + Ca++)-induced release, no decay in the responsiveness to Ca++ is observed after PS addition. Both the antigen-induced and non-antigen-induced release are completed fairly rapidly and are dependent of temperature, pH and energy.