A recently described chimaeric organ culture system is employed to investigate the effects of antibodies to the CD3 and CD2 cell surface structures on the fate of developing human thymocyte populations. Engagement of CD3 as well as CD2 with reagents that stimulate peripheral human T cell results in the activation-induced death of human thymocytes in organ culture. Death is preceded by a characteristic phenotypic change; thymocytes that bear CD45RA (determinants associated with high-molecular-mass isoforms of the leukocyte common antigen family, CD45) are first induced to co-express CD45RO (the low-molecular-mass isoform of CD45), and subsequently lose CD45RA expression. This antigenic change is followed by cellular DNA fragmentation, characteristic of apoptosis. We conclude that engagement of CD3 as well as CD2 can recruit human CD45RA+ thymocytes into the CD45RO+ population, where cell death occurs. Our results suggest that this phenotype conversion is a marker for thymocytes destined for programmed cell death.