The antimicrobial activity of ampicillin, azlocillin, cefotaxime, cephapirin, clindamycin, mezlocillin, nafcillin, piperacillin, tobramycin, and vancomycin was tested in peritoneal dialysate at room temperature for 24 hours and 48 hours. All of the antimicrobial agents were active at 24 hours. The bioactivity of cefotaxime, nafcillin, and vancomycin declined 15% to 20% after 48 hours (P less than 0.001). The addition of heparin or insulin did not affect the activity of any of the study drugs. The combination of cephapirin and tobramycin remained bioactive for 24 hours at room temperature and 35 degrees C. The preparation of drug-dialysate solution every one to two days is feasible for the treatment of peritonitis in patients on continuous ambulatory or continuous cycled peritoneal dialysis.