Upon immunization with phosphorylcholine (PC), BALB/c mice produce antibodies bearing mainly one idiotype which is identical to that of the TEPC 15 PC-binding myeloma protein. By coating sheep red cells (SRC) with TEPC 15 it was possible to detect anti-idiotypic plaques during the course of an immune response to PC. These plaques are detectable only after the anti-PC response has reached a peak and begun to decline. They are specific, since addition of free TEPC 15 and not MOPC 315 prevented their detection. It is suggested that such anti-idiotypic reactive cells might regulate the response to PC.