Working with the antimetabolic chemopredictivity assay on short term cultures we evidenced a correlation between in vitro chemosensitivity of colorectal cancer and fractional incorporation of radionucleotides. Four different drugs (5-FU, mitomycin C, 4’-iododeoxydoxorubicin and ruboxyl, a nitroxyl derivative of daunorubicin) were tested on 102 tumor cultures. The extent of 3H-TdR and 3H-UdR incorporation into DNA and RNA in the related control cultures was in relationship with the chemosensitivity of the tumor. Along with the labeling index this simple metabolic-kinetic trait may gain a role for the screening of tumor phenotypes sensitive to chemotherap