SummaryThe production of gamma-interferon (y-IFN) and antigen-speeific immunoglobulin isotypes was monitored in sheep given primarj' and seeondary inocula of protein and polysaccharide antigens with or without adjuvants. In efferent lymph from prefemoral nodes draining the site of inoculation, y-IFN levels increased within 24 h after injection of adjuvants Quill A. dextran sultate (DXS) and 6 days after oil adjuvants in saline. The increased synthesis of y-lFN was prolonged by 1-2 days by the presence of adjuvanted antigen, but the major stimulus to y-IFN production was provided by the adjuvant. Alhydrogel (AH) did not initiate y-IFN synthesis. Following a second inocula of antigen in saline, increased levels of y-lFN were detected in efferent lymph after 3-4 h, and persisted for at least 2 days. Following cultivation ofleucocytes from immunized sheep with specific antigen in vitro, strong positive correlations between levels of interleukin (IL) IL-2, y-lFN, proliferative responses and antibody titres were observed. However, after analysisof the antigen-specitic isotypesof immunoglobulin (Ig) there was no correlation between the production of-y-lFN and particular Ig isotypes. Together with findings that AH did not preferentially induce IgGl or IgG2. these results suggest that the specific subpopulations of helper T lymphocytes which regulate the production of antibody isotypes by differential secretion of y-lFN or IL-4, are not as clearly defined in sheep as in mice.