Calyculin A-induced premature chromosome condensation (PCC) assay is considered a method for estimating radiation dose. In this study, a novel parameter, the transformed cell ratio (TCR), which is the ratio of irradiated and unirradiated lymphocyte transformation rates in the PCC assay, was suggested for dose estimation since it may be correlated with ionizing radiation dose. Human peripheral lymphocytes were irradiated in vitro with 60Co gamma ray doses of 0, 10, 20, 30, and 40 Gy, stimulated with phytohemagglutinin, and incubated for 3, 6, 12, or 48 h, division-blocked with colcemid, and PCC-induced by calyculin A or addition of extra cyclin-dependent kinase 1/cyclin B (CDK1/cyclinB). Results showed that G1-, S-, G2-, and M-PCC cells appeared in human lymphocyte culture, dose-response curves between the absorbed dose and lymphocyte transformation rate or TCR was fitted. TCR presented the best goodness-of-fit to a linear model (y = 0.0097 X +0.9952, R2 = 0.971, P < 0.05) within 0 to 40 Gy. Observations also suggested that TCR is a potential biological dosimeter for assessing received radiation doses ranging from 0 to 40 Gy during radiation exposure.