Peritoneal mast cells of 4 inbred strains of mice (BALB/c, 129, C3H/A, C57BL/6J) and F1 hybrids of August and Wistar rats were studied for their sensitivity to the action of anti-IgE antibody for histamine release. The mast cells from 4 mouse strains tested showed different ability to release histamine after challenge with several anti-IgE dilutions; mast cells of BALB/c mice showed the lowest anti-IgE reaction. After active sensitization of mice and rats their mast cells release histamine after challenge with specific antigen, however, detectable changes in their anti-IgE sensitivity were not observed. After passive sensitization of mice the mast cells of 3 of 4 strains tested showed slightly enhanced anti-IgE sensitivity for histamine release. The sensitization of mast cells does not always stimulate anti-IgE in reversed anaphylaxis.