The feasibility and reliability of an in vitro assay that evaluates drug interference on nucleic acid precursor incorporation were investigated on 135 previously untreated locally advanced breast cancers. The assay, which was carried out on tumor fragments incubated for 3 hours with drugs, proved to be feasible on a sufficiently high percentage of biopsy specimens (70%) for routine clinical use. In vitro drug activity evaluated with this assay appeared to reproduce the clinical patterns of sensitivity of the tumor type as well as of the individual tumors. In fact, in vitro response rates to conventional agents resembled the clinical response rates reported for the same agents used in monochemotherapy. From a retrospective--correlative study carried out on 41 patients treated in vitro and in vivo with the same drugs (Adriamycin [doxorubicin] and vincristine), in vitro effect of Adriamycin on 3H-uridine incorporation appeared significantly correlated with clinical response (overall agreement, 78%; P = 0.0032) with specific agreements of sensitivity and resistance of 75% and 81%, respectively.