Abstract: A method for quantitative propagation of hemotropic parasites, especially Babesia parasites, in vitro, Babesia-infected erythrocyte cultures are incubated in an environment of enhanced carbon dioxide tension wherein the culture medium overlay provides a barrier to oxygen transmission such that cellular hemoglobin is essentially maintained in its deoxy state. By varying environmental carbon dioxide tension, merozoite stages of the parasite are selectively reversibly induced to migration between residence in host cells or culture medium, rendering possible the isolation of infective antigenic merozoites in large quantities. Specific Babesia antigen is harvested in large quantity from the in vitro culture medium.