Development of the antigen-specific murine T cell response to immunization with keyhole limpet hemocyanin (KLH) in adjuvant has been monitored with direct limiting dilution analysis of CD4+ cells in draining lymph nodes (LN) and measurement of the cytoldnes produced by their clonal progeny. In vivo, the response to immunization suggested a major role for IL-4 and a minor role for IFN-γ since IL-4 mRNA levels increased and IFN-γ mRNA levels declined in LN over the first 3 days, and KLH-specrflc serum antibodies were mainly of lgG1 class with lower levels of IgE and lgG2a. Antigen-specific clonogenlc cells were first detected in LN at day 4, at which time they comprised - 8% of the total CD4+ LN cell pool, declining to 1-2% from day 7 until at least 6 weeks after immunization. These clonogenlc cells expressed high levels of surface CD44 (Pgp-1) both early and late in the in vivo response. Over the whole of the time span from day 4 to 6 weeks after immunization, most antigen-specific cells gave rise to clones that secreted IL-4 and a smaller proportion gave rise to IFN-γ-secreting clones. By contrast polyclonally activated CD4+ cells from untreated mice preferentially gave rise to clones with the converse cytoklne profile. We conclude that a stable ratio of antigen-specific CD4+ cells committed to IL-4 or IFN-γ synthesis is established within the first 4 days after KLH immunization and, contrary to prediction, does not evolve towards a more restricted cytoklne profile during the primary response.