1.An examination of 309 sera from human cases of malaria taken at various intervals following the attack, indicates that the monkey antigen (P. knowlesi) as prepared by Coggeshall gives specific positive complement fixation reactions. The particular antigen we used was effective in a dilution of 1:160 and also 1:320.
2.The time of appearance and the concentration of the antibody responsible for the reactions varies with the number of attacks. As a rule the reaction becomes positive during the third week following the attack, provided that the patient has had two or more attacks. Cases cured after one attack failed to give a positive reaction.
3.In the cases which we have been able to follow continuously the reaction disappeared again during the fourth month following the last attack.
4.The results indicate that the reaction may prove useful for epidemiologic purposes. Mass tests carried out in a hyperendemic area yielded the following results: (a) About 98 per cent of the sera of children up to the age of twelve gave positive reactions whether or not they showed a blood infection at the time of the test. (b) Infected adults in this area gave similar results. (c) Healthy adults in this area gave only 10 percent positive reactions. (d) Sera of persons who have been subject to repeated treatment give positive or negative reactions according to the interval between the last attack and the date of the test.
5.Preliminary experiments show that there is cross reaction between the antigens of P. knowlesi and P. gallinaceum and sera from a monkey and chicken immune to their specific parasite. Furthermore, the P. gallinaceum antigen gave positive complement fixation with such sera from human cases of malaria as reacted with the P. knowlesi antigen.