We examined influences of certain antibiotics on the release of chemical mediators from isolated rat peritoneal mast cells in vitro.Isolated peritoneal mast cells were obtained from male Wistar strain rats.Everninomicin(0.06−1.2 mg/ml), vancomycin(0.05−1.0 mg/ml), teicoplanin(0.07−1.4 mg/ml)and concanavalin A(0.01 mg/ml)were used.Isolated mast cells were incubated in the presence of various concentrations of the test compound at 37°C for 10 min.Histamine contents of the supernatant and cell pellet were measured by an automated fluorometric method.Prostaglandin D₂(PGD₂)contents of the supernatant were determined by enzyme immunoassay.Everninomicin(0.06−1.2 mg/ml)had no influence on histamine and PGD₂ release from mast cells.On the other hand, vancomycin significantly released both histamine(0.5 and 1.0 mg/ml)and PGD₂(1.0 mg/ml)from mast cells, but vancomycin did not affect concanavalin A−induced histamine and PGD₂ release.Teicoplanin(more than 0.07 mg/ml)significantly stimulated histamine and PGD₂ release from mast cells and it also significantly potentiated concanavalin A−induced histamine and PGD₂ release.These results suggest that everninomicin causes no chemical mediator release from mast cells, different from vancomycin and teicoplanin.