A short-term antimetabolic assay based upon the inhibition of incorporation of nucleic acid precursors was used to compare the cytotoxicity of a new halogenated anthracycline, 4'-iodo-4'-deoxydoxorubicin (IDX), with that of its parent compound doxorubicin (DX) on human colo-rectal carcinoma specimens. IDX showed a marked dose-dependent effect, with frequencies of activity consistently greater than those of DX at all concentrations. The minimal dose required to induce a significant antimetabolic effect for IDX was 1/10 that for DX.