Proliferative rates of 73 node-negative adenocarcinomas of breast with 5-year or greater follow-up were studied using cytokeratin staining in two-parameter DNA flow cytometry of archival specimens. Quality control data of accuracy of the measurements were determined and all analyses were compared with single-parameter results of the same specimens, using demarcated tumor areas, quadruple analyses, and computerized nonspecific staining subtraction. Mitotic rates of the same samples correlated highly significantly with the S-phase fractions and proliferative index (S + G2 + M phases), especially for the cytokeratin data. The predictive value of mitotic rates was found significant, but that of the DNA flow-cytometry-obtained indices was not, probably because of low numbers of deaths in this study. The cytokeratin method identified heteroploid tumors containing a diploid cell population not identifiable by single-parameter analysis. In conclusion, cytokeratin staining can be reliably applied to DNA flow cytometry of archival specimens giving accurate ploidy, S-phase fractions, and proliferative index data limited almost exclusively to neoplastic cell populations. This will permit large-scale retrospective studies aimed at establishing the usefulness of DNA flow cytometry for clinical decisions on therapy of surgically removed node-negative adenocarcinomas of breast.