Ia. W39 is a B cell differentiation antigen whose membrane expression is controlled by the xid gene. In this report we show that, analogous to its B cell expression, Ia. W39 is also present on a subset of Ia+ macrophages, indicating heterogeneity within that cell population. The Ir gene(s) for the antigenic determinants on the A chain loop of beef insulin maps to the I-Ab subregion of the H-2 complex and, as we have previously reported, is associated with the private specificity Ia. W39. Depletion of Ia. W39+ macrophages eliminates their capacity to present beef insulin to immune T cells, whereas the presentation of the multideterminant antigen trinitrophenylated ovalbumin is reduced less than 50%. Furthermore, we found that H-2b mice lacking Ia. W39+ cells are unable to make a secondary in vivo IgG plaque-forming cell (PFC) response to beef insulin, while the primary IgG PFC response is not dependent on Ia. W39. No shift in the kinetics of the response, nor development of suppressor T cells could be detected in Ia. W39- mice, which would explain their apparent nonresponsiveness to beef insulin after boosting with this antigen. These results, therefore, may reflect a difference in the Ir gene control acting at the primary vs. secondary response level.