The L49 monoclonal antibody against the p97 antigen on melanomas and carcinomas was chemically conjugated to E. coli beta-galactosidase (beta-gal), forming a largely monomeric conjugate with preserved enzymatic activity. The resulting L49-beta-gal conjugate was used to activate (N-[(4"R,S)-4"-hexyloxy-4"-(1'''-O-beta-D-galactopyranosyl)butyl]daunorubicin) (1), a derivative of daunorubicin that has low cytotoxicity and high chemical stability. Addition of the conjugate to the prodrug resulted in an increase in cytotoxicity of approximately 10(5)-fold, a level of activation that is higher than any mAb-enzyme/prodrug combination yet described. Furthermore, the released drug had an IC(50) value of approximately 10 pM, making it significantly more potent than the vast majority of clinically approved anticancer drugs. The potential of this enzyme/prodrug combination for cancer therapy is discussed.