CD45RA-RO+ subset is the major population of dividing thymocytes in the human CD45, or leukocyte common antigen, is expressed in different isoforms on different subsets of thymocytes, suggesting its involvement in the process of T cell development in the thymus. We report studies on CD45 isoform expression on human thymocytes at various stages of development using three-color flow cytometric analysis and cell cycle analysis. Among CD45ROf cells 18.4% were in S+G2/M phase and represented more than 80% of the dividing cells in the thymus. Among the CD45RO-cells 10.9% were also in cell cycle. Because the CD45ROt population is almost exclusively CD45RA-, the CD45RA-R0+ subset constitutes the major portion of dividingcells in the thymus. Both the CDlhigh and the CD3-populations were actively cycling. However, the former was almost 100% and the latter only 50% CD4SRA-ROf. Dividing CD4SRA-R0+ cells contain, therefore, many cells that have not yet expressed the CDYT cell receptor complex and presumably have not yet undergone selective procedures. These results are hard to reconcile with the previously presented hypothesis that CD4SRO represents a marker for cells that are destined to die in the thymus. Instead, these results suggest an alternative possibility that CD45ROf cells may contain cells that can mature and, thus, also constitute the thymic generative lineage.