The proliferative response of hapten-reactive guinea pig T lymphocytes to TNP-modified macrophages is regulated by the I-region of the MHC of the priming macrophage and can be specifically inhibited by both anti-TNP serum and anti-Ia serum directed against the stimulator macrophage. Although these findings suggest an intimate association of macrophage Ia antigens and the TNP-specific immunogen, the precise nature of the antigenic determinant(s) recognized by the T cell remains unclear. We have investigated this apparent association by chemical methods. Peritoneal exudate cells from strain 13 guinea pigs were radio-labeled with 125Iodine or 3H-leucine, derivatized with trinitrobenzene sulfonate, and lysed with detergent. TNP-modified proteins were then precipitated with anti-TNP serum and analyzed by SDS-polyacrylamide gel electrophoresis. Although multiple membrane proteins were precipitated by anti-TNP serum, sequential precipitation experiments using anti-TNP or anti-Ia sera demonstrated that the TNP-bearing molecules and Ia antigens were distinct and separate from one another.