Abstract A decrease in thymus function has been observed in aging individuals. This may be due to changes either in the thymocyte population or in the thymus microenvironment. A function of the latter is secretion of thymic factors, one of which is human thymic epithelial-conditioned medium (HTECM). In this study we have investigated the responsiveness of human thymocytes from individuals of different age to mitogens and the effect of HTECM upon this response. The responsiveness of human thymocytes to Con A and PHA increased with advancing age, and was most prominent for PHA. This increase could not be attributed to changes in monocyte numbers. Thymocytes sensitive to HTECM modulation with Con A as mitogen were found in all age groups. With PHA as mitogen only thymocytes from individuals under the age of 8 years were sensitive to HTECM. The results can be explained by a relative increase of the more mature, medullary thymocytes in the thymus with age.