1 The metabolism of cyclophosphamide was investigated before and following treatment with chloramphenicol (five subject) and sulphaphenazole (seven subjects). 2 The biotransformation of cyclophosphamide was retarded after chloramphenicol treatment as indicated by prolongation in the mean serum half-life from 7.5 h to 11.5 h combined with decreased peak activity and integrated concentrations of metabolites. 3 In the sulphaphenazole treated subjects the rate of biotransformation of cyclophosphamide was significantly inhibited in two, unaltered in three and enhanced in two. The inconsistency of the results is discussed.