The use of specific T helper cells (TH) has allowed the detection of anti-hapten plus self cytotoxic T-lymphocyte (CTL) precursors in cell populations unable to generate autonomous reactions (and considered as 'immature'). This led us to reevaluate the ontogeny of CTL precursors in different tissues in the presence of adult TH. It was found that thymocytes from newborn mice contained levels of CTL precursors comparable to those of adults and were dependent on TH throughout the donor's life. In the spleens, however, 3 periods could be distinguished: (a) from birth to 4 days after birth, no CTL precursors could be detected even in the presence of TH; (b) from the 5th day to the 2nd week after birth, CTL responses could be obtained only in the presence of TH; (c) after the 2nd week, autonomous responses could be obtained. No evidence for a suppressive mechanism was found in the spleen of the newborn mice, nor could lack of responsiveness be attributed to newborn accessory-cell malfunction. The data indicate that a limiting cell in the development of anti-hapten plus self CTL is the TH from the 4th to the 16th day after birth. The absence of CTL precursors in the spleen, but not in the thymus of neonates would indicate differential T-cell maturation in the two organs or late migration to the spleen.