The compatibility of cefazolin and gentamicin in fluid commonly used for continuous ambulatory peritoneal dialysis (CAPD) was studied. Five admixtures containing cefazolin (75 mg/L and 150 mg/L) and gentamicin (8 mg/L), alone and in combination, were prepared in 1.5% dextrose peritoneal dialysis solution. Solutions were stored for 48 hours at 4 degrees C, 26 degrees C, and 37 degrees C; aliquots for drug assay were obtained at 0, 4, 8, 24, and 48 hours. HPLC and immunofluorescent assays were used to determine cefazolin and gentamicin concentrations, respectively. The cefazolin and gentamicin concentration changes over the study period did not reach statistical significance. Maximal cefazolin and gentamicin losses (12 and 7 percent of the initial concentrations, respectively) were observed at 48 hours in solutions stored at 37 degrees C. No significant differences in concentration changes were observed between combination solutions and solutions containing either cefazolin or gentamicin alone. Cefazolin and gentamicin, alone or in combination, are compatible for at least 48 hours in CAPD solutions.