By using affinity-purified ookinete surface antigen from the rodent malaria parasite Plasmodium berghei, a transmission-blocking immunity was induced in mice. Groups of mice were immunized via different routes, with total quantities of antigen ranging from 0.5 to 40 micrograms (with or without Freund adjuvant). Vaccination by the intramuscular route with 20 micrograms of antigen in the absence of adjuvant and boosted once with the same amount of protein induced a total transmission blockade. Immunoblot analysis confirmed that immune sera invariably recognized Pbs21 antigen. The isotype and titer of the anti-Pbs21 immunoglobulin G (IgG) response was determined by enzyme-linked immunosorbent assay. The antibody isotype was predominantly IgG1. The concentration of specific anti-Pbs21 IgG reached a peak of 182.45 +/- 92.13 micrograms/ml by week 7 postimmunization and fell progressively to 38 micrograms/ml at week 34 (at which time the transmission was still inhibited by 98%).