Fading of immunity in Swiss and C3H/StZ mice was progressive and was related to the interval elapsed since in the last challenge infection and survival of parasites [Plasmodium berghei]. In both strains the proportion of mice that remained immune to challenge decreased with increasing fading periods. A shift from delayed mortality to a normal course of infection as seen in non-immune controls was observed in C3H/StZ mice. Early parameters of fading were increasing peak parasitemias after challenge in an increasing proportion of mice, and lethal infections instead of transient parasitemias. Fading was reflected by changes in the host cells preferentially infected early after reinfection: with increasing fading periods host cell preference shifted from predominantly polychromatophilic erythrocytes to mixed infections, and finally to predominantly oxyphilic cells, especially in Swiss mice. The results of isodiagnosis indicated a positive correlation between persistence of parasites and immunity, whereas the absence of parasites was related to various phases of a fading immune response.