With strict adherence to ethical guidelines, a volunteer was immunized against sporozoites of Plasmodium falciparum and P. vivax, the antigen consisting of attenuated sporozoites of each species inoculated through bites of mosquitoes X-irradiated at a minimum dosage of 15,000 rads. On one occasion this dosage did not render all P. vivax sporozoites noninfective. Species specificity of antigen and antibody was demonstrated, but within each species a wide geographical diversity of strains proved interchangeably antigenic and susceptible to the antibody. Once immunized, the volunteer was protected for not more than 3 months and 6 months, respectively, from infective P. falciparum and P. vivax sporozoites, the duration of protection being reflected by a positive species-specific circumsporozoite reaction. Studies in this volunteer, and in two others immunized with P. falciparum sporozoites, did not reveal any increase in serum levels of immunoglobulins G and M.