Purification of mast cells from the peritoneal cavity of rats and reactivity of the cells to antigen were studies. After peritoneal lavage, the peritoneal cells were washed, the mast cells were isolated by a density gradient centrifugation method. The recovery rate of mast cells was highest when mast cells were separated by BSA density gradient centrifugation after washing the cells on time by low speed centrifugation (50xg). The purity of mast cells obtained by this method was 94.1±1.0%, and the number of mast cells obtained was approximately 1.46×10(6) cells/rat. Mast cells of actively sensitized rats showed a significant increase in (45)Ca uptake and histamine release after stimulation by antigen. (45)Ca uptake by mast cells of passively sensitized rats was affected by BSA medium, and the most suitable results were obtained using mast cells passively sensitized before separation of the cells by BSA. Mast cells showed a significant increase in (45)Ca uptake after stimulation by various stimulatory agents, such as Con. A, Comp. 48/80 and Ca ionophore A 23187. These results indicate that rat peritoneal mast cells isolated by the present method can be used in studies on Ca(2+) influx into the cells.