Objective To observe the inhibitory effect of the antibodies against midgut-protein-ingredient of Anopheles stephensi on the oocysts of Plasmodium yoelii.Methods Female An.stephensi mosquitoes raised in laboratory were dissected and the midguts were collected.Eight BALB/c mice were immunized using midgut-protein(100 ?g/mouse,4 times with an interval of 7～10day).Ten days after the last immunization,blood was taken from mice armpit artery and serum separated.The immune active antigen of the midgut protein was analyzed by Western blotting.Protein with Mr 38 000～50 000 was separated by sephadex filtering and used to immunize 12 BALB/c mice(100 ?g/mouse,4 times with interval of 7～10 days).PBS control group was established.Seven days after the last immunization,serum antibody was detected by ELISA.When the antibody titer in immunized mice reached ≥1:2 560,mice in both groups were infected by P.yoelii(about 2?107 plasmodium-infected RBC) by abdominal injection.The mosquitoes were fed on the infected mice when the number of female gametes was higher than 2 per 10 microscopical fields 3 days later.After 9 days,the mosquitoes were dissected and the amount of oocysts in midgut was counted.Results Eight protein bands were shown in midgut-protein of An.stephensi by Western blotting and the band of Mr 38 000～50 000-midgut-protein appeared clearer.The infection rate of oocysts in the experiment and control groups were 28.70%(62/216) and 51.09%(47/92) respectively(P