A void volume fraction and fractions of mean sizes 800 kdalton and 300 kdalton were isolated by gel filtration from lysate of bovine erythrocytes infected with Babesia bovis. All fractions had good serological activity, as assayed by ELISA and IFA. Groups of four splenectomized calves were vaccinated with each fraction and then challenged, together with groups of four control calves, with a homologous strain of B. bovis. The group vaccinated with the 300 kdalton fraction showed some protection, as indicated by delayed and significantly lower parasitaemias and by a 75% survival in the group. In contrast, all animals in the relevant control group died from infection. No evident protection was obtained with the other two fractions.