To understand better the repopulation kinetics of tumor cells after radiotherapy, we investigated changes in cell proliferative parameters after single-dose irradiation of SCCVII tumors in C3H/He mice and EMT6 tumors in Balb/c mice. The following parameters were determined 0-15 days after single irradiation at 20 or 30 Gy; dividing fraction (DF), potential doubling time (Tpot), number of clonogenic cells per tumor (Ncln), and volume doubling time (Tvol). DF and Tpot were determined by in vivo-in vitro cytokinesis-block assay with cytochalasin B, Ncln was measured by in vivo-in vitro colony-forming assay, and Tvol was determined by growth delay assay. In both tumors, longer Tpot and lower DF and Ncln were obtained for 3-4 days after irradiation, but in SCCVII tumors these values returned to the pretreatment levels 9 days after irradiation. In EMT6 tumors, Tpot, DF, and Ncln did not return to the pretreatment levels even 12 days after irradiation. In the regrowth phase of both tumors following irradiation at 20 Gy, Tvol was longer than the pretreatment level, although Tpot was similar in SCCVII and only slightly longer in EMT6. Therefore, the cell loss factor in the regrowth phase was considered to be higher than the pretreatment level in both tumors. From the results, recruitment of previously quiescent cells into the proliferative pool in these tumors was suggested to contribute to repopulation after radiation.