A Plasmodium gallinaceum parasite preparation was made. The parasites in the preparation were morphologically intact and, when examined microscopically, appeared free of host materials. Serological tests indicated that this parasite preparation was agglutinable by plasmas from chickens with a history of P. gallinaceum infection but not by plasmas from chickens with no history of infection. Inactivation of the plasma at 50°C for 30 minutes did not cause significant changes in the ability of the plasmas to agglutinate the parasites; however, heating for one hour at 56°C reduced or abolished the agglutination reaction. A factor of considerable importance in the agglutination reaction was the parasite concentration in the antigen. For use in the capillary agglutination test the antigen was adjusted to contain one part of packed parasites and 4 parts of pH 7.2 phosphate buffered saline.
The antigen was stable at refrigerator temperature for 85 days, which was the longest period tested.