Nematospiroides dubius, Nippostrongylus brasiliensis and Trichuris muris infections in C57BL mice increased background IgM titres to SRC but not to erythrocytes from other species. However, no cross-reaction between parasite extracts and SRC could be demonstrated and the enhancement was not evident in other strains of mice. There were no comparable rises in total IgM levels, but declining IgM responses to ovalbumin were boosted by N. dubius, so the rise was probably caused by selective expansion of activated clones of cells. N. dubius also increased responses to low doses of SRC given intraperitoneally (i.p.) a few days after infection, whilst optimal SRC doses induced normal responses. In contrast, N. brasiliensis and T. muris often depressed responses to SRC though the effect was variable and the timing of challenge was critical. Thus, whereas the enhancement of natural titres occurs in C57BL mice infected with any of the parasites tested here or in previous studies, the effects on responses to injected SRC depend on the species of parasite.