The role of antibodies in the neutralization of sporozoites of Theileria parva was investigated. It was found that serum obtained from cattle recovered from East Coast fever (ECF) and rabbits immunized with sporozoites was capable of neutralizing the parasites. The serum was then fractionated by gel filtration and anion exchange chromatography to identify which antibody class or subclass was responsible for the neutralization. In bovine hyperimmune serum the neutralizing capacity was in the fraction containing IgG2 immunoglobulins. In rabbit anti-sporozoite serum the neutralizing capacity also resided in the IgG2 containing fraction. These results suggest that humoral antibodies may play a role in resistance to reinfection with T. parva. A mechanism for this acquired resistance is proposed based upon the recently established biological properties of bovine IgG2 immunoglobulins.