A two component system, consisting of a fusion protein and an appropriate prodrug, suited to perform selective tumor therapy in vivo is presented. The fusion protein, due to its humanized carcinoembryonic antigen-specific variable region, specifically binds to carcinoembryonic antigen-expressing tumors and has an enzymatic activity comparable to that of human beta-glucuronidase. The prodrug is a nontoxic glucuronide-spacer derivative of doxorubicin decomposing to doxorubicin by enzymatic deglucuronidation. In vivo studies in nude mice bearing human carcinoembryonic antigen-expressing tumor xenografts revealed that 7 days after injection of 20 mg/kg fusion protein a high specificity ratio (> 100:1) was obtained between tumor and plasma or tumor and normal tissues. Injection of 250 mg/kg of prodrug at day 7 resulted in tumor therapeutic effects superior to those of conventional chemotherapy without any detectable toxicity. These superior therapeutic effects which were observed using established human tumor xenografts can be explained by the approximately 4-12-fold higher doxorubicin concentrations found in tumors of mice treated with fusion protein and prodrug than in those treated with the maximal tolerable dose of drug alone. The nondetectable toxicity in the animals treated with fusion protein and prodrug is probably caused by up to 5-fold lower drug concentrations in normal tissues compared to the animals treated with doxorubicin. Thus, a more tumor-selective therapy, resulting in stronger therapeutic effects and reduced toxicity seems to be possible by the appropriate use of the humanized nontoxic fusion protein and the nontoxic prodrug.