Phenotypic analysis of the medullary-type CD4+ CD8- [CD4 single-positive (SP)] thymocytes has revealed phenotypic heterogeneity within this cell population. The characteristic phenotype of mature peripheral T cells can be uniquely marked as Qa-2+ HSA- CD69-, whereas in the medullary-type CD4 SP thymocytes, the expression patterns of many markers were quite different. This suggests that there are many subgroups in the population, which reflects that medullary-type CD4 SP thymocytes may undergo phenotypic maturation. According to the results of two-colour flow cytometry, seven discrete phenotypes were identified by the expression capacity of Qa-2, HSA, CD69, 3G11 and 6C10 molecules. Consequently, the phenotypic precursor-progeny relationship can be envisaged as: 3G11- 6C10+ CD69+ HSAhi -->3G11+ 6C10+ CD69+ HSAhi --> 3G11+ 6C10- CD69+ HSAint -->3G11+ 6C10- CD69- HSAint Qa-2- -->3G11+ HSA-/lo Qa-2lo. At the stage of 3G11+ 6C10- CD69- HSAint Qa-2-, a branch pathway could be initiated, which gave rise to 3G11- HSAl degrees Qa-2- cells, which then, in turn, developed into 3G11- HSA-/loQa-2hi cells, a minor subgroup of the most mature CD4 SP cells. Consistent with this predicted pathway, experiments indicated that the first two subgroups were still cortisone sensitive, whereas the others were cortisone resistant. The cells in the last two Qa-2-positive subgroups are probably ready for emigration into the periphery.