The results of studies on the effect of a new synthetic antigens: interpolymer of styrene and maleic acid (PSM) on the immunological response in rats to sheep red blood cells, and vice versa are presented. It was found that in rats, in contrast to mice, PSM did not generate antigen-binding cells for sheep red blood cells. However, an opposite effect was observed: sheep red blood cells generated in rats antigen-binding cells to PSM.