The ontogeny of interleukin 2 (IL 2) production in young CBA/HT6T6J mice was studied using both bulk culture and limiting dilution methods. The ability of spleen cells in bulk culture to produce IL 2 in response to concanavalin A (Con A) was found to rise through the first 2 weeks of life, from no production at day 1, through 20 units/ml at day 6, to 80-100 units/ml in adults. No evidence for suppression of IL 2 production by young spleen was found. Limiting dilution analysis of both young spleen and young lymph node (LN) shows that young spleen has a much lower complement of cells producing IL 2 in response to Con A or allogeneic stimulation than does adult spleen. The frequency of 6-day spleen cells producing IL 2 in response to Con A is 1/1000, while the adult frequency is approximately 1/50. Young LN, in contrast, has levels of IL 2-producing cells close to those of adult LN, with a frequency of responders to Con A of 1/20. No evidence was found for a deficiency in IL 2 production on a per cell basis, in either 6-day spleen or LN. In examining allogeneic reactivity, a high frequency of cells reacting to strong Mls stimulation was found in both young and adult spleen and LN.