The chemosensitivity of 26 non-pretreated colorectal carcinomas (primary tumors and/or colorectal metastases) was studied by an in vitro antimetabolic assay, which evaluates the interference of drugs on the incorporation of 3H-thymidine and 3H-uridine in short-term cultures of human tumors. Our results correlate with the response rate obtained in clinical studies with monochemotherapy and justify the possibility of a future prospective study using individually tailored chemotherapy regimens. Doxorubicin-analogues, with an overall in vitro efficacy in 16.0% and 14.3% for 4-epidoxorubicin (epi-DX) and 4-deoxydoxorubicin (deo-DX), respectively, seem to deserve a modest role in the treatment of colorectal cancer, provided that a careful selection of patients is performed. Variability in anthracycline activity is indeed evident, also in our study, in relation to the different neoplastic picture of the various patients.