Serum was obtained from CBA/Ca mice infected, reinfected or superinfected with parasites taken one or two syringe passages from cryopreserved reference stabilates derived from cloned lines of the AS or CB isolates of P.c. chabaudi. Serum was also collected from mice superinfected with parasites derived from a cloned line of P. berghei KSP-11. When injected into normal syngeneic recipients subsequently challenged with homologous or heterologous parasites, these sera mediated some or all of the following modifications to the breakthrough parasitaemias which invariably occurred (i) an extension of the pre-patent period (ii) an extension of the time taken for the parasitaemia to reach 2% (iii) a reduction of peak parasitaemia (iv) protraction of the initial peak of parasitaemia. These modifications were particularly evident with serum from superinfected mice and to a lesser extent with serum from animals reinfected once after recovery from a primary infection. Serum taken during the course of such a primary infection produced extended pre-2% periods, other effects being only marginal. Serum mediated modifications produced by reinfection and superinfection serum appeared largely species-specific with a limited degree of cross-reactivity. Intraspecific specificity was also apparent with serum from P.c. chabaudi AS or CB reinfected or superinfected mice, although marginal cross-immunity was again observed. When analysed by the fluorescent antibody technique on smears of methanol fixed parasitized erythrocytes, reinfection and superinfection sera were almost totally cross-reactive both within and across species. Preliminary evidence that parasites breaking through the effects of these sera may constitute a phenotypic antigenic variant is presented and possible mechanisms for the parasitaemia modifying effects of the various sera discussed.