The ontogeny of interleukin 2 (IL2) production in young CBAIHT6T6.l mice was studied using both bulk culture and limiting dilution methods. The ability of spleen cells in bulk culture to produce IL2 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 unitdm1 in adults. No evidence for suppression of IL2 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 IL2 in response to Con A or allogeneic stimulation than does adult spleen. The frequency of 6-day spleen cells producing I L 2 in response to Con A is 1/1000, while the adult frequency is approximately 1/50. Young LN, in contrast, has levels of ILZproducing 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 I L 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.