Hepatozoon canis is a tick-borne apicomplexan parasite of dogs that infects neutrophils and parenchymal tissues. To study the antigenic characteristics of this parasite, a technique was devised for the purification of gamonts from peripheral blood neutrophils. White blood cells were separated on Ficoll-Hypaque density gradients and the gamonts were released from the host neutrophils by nitrogen cavitation. The blood used for purification originated from dogs with natural or experimental infections of H. canis with a parasitemia of 1.4-33%. The number of parasites collected ranged from 1.5 X 10(6) to 4.2 X 10(7). Portions of purified gamonts were separated and examined under phase and scanning electron microscopy, and the remaining purified parasites were then used as a source of antigens to characterize the humoral immune response by western blot analysis. Serum antibodies from infected dogs recognized more than 15 gamont antigens, and the antigenic patterns observed with sera from naturally and experimentally infected dogs were nearly similar. Four immunodominant protein bands of relative molecular weights of 107, 88, 63, and 28 kDa were recognized by all of the sera examined. The technique applied here for the isolation of host cell-free gamonts will facilitate studies on antigenic composition and immune responses against H. canis and on antigenic relationships between Hepatozoon from different host species and geographic regions.