The hybrid synthetic protein SPf(66), which contains small fragments of the 83-kD, 55-kD, 35-kD, and circumsporozoite antigens of Plasmodium falciparum, was studied to determine its protective capacity against malaria infection in Aotus lemurinus monkeys. Two groups of six monkeys each were immunized six times with this polymer, which was mixed with either Freund's adjuvant or aluminum hydroxide. Two groups of five animals each were used as controls and immunized with saline solution mixed with the same adjuvants. Neither antipeptide nor antimalarial antibodies developed after the six immunization doses. Regardless of this fact, the monkeys were challenged intravenously with 105P. falciparum blood stage parasites, and the resultant parasitemia was followed daily on blood smears. Only one monkey from each of the groups immunized using Freund's adjuvant (both experimental and control) was protected. In those immunized using aluminum hydroxide, one animal was protected in the experimental group, but none were protected in the control group. Anti-parasite antibodies developed during the infection, but did not correlate with protection and failed to recognize SPf(66) peptide in an enzyme-linked immunosorbent assay. Immunization with the polymer did not boost natural antibodies present in two of the monkeys before the experiment. Low levels of gamma-interferon were produced in some animals, but were not correlated with protection.