Cytokine responses in human host-protective immunity to malaria have yet to be completely elucidated. No data appear to exist on the cytokine patterns in non-human primate models immunized with malarial antigens. Expression of mRNA transcripts of 10 cytokines, the adhesion molecule ICAM-1 and inducible nitric oxide synthase (iNOS) in peripheral-blood mononuclear cells (PBMC) from nine Aotus monkeys was analysed by reverse-transcriptase PCR. Five of the monkeys had been immunized with multiple-antigen peptides (MAP) of the Plasmodium vivax circumsporozoite protein and two with constructs of the P. falciparum merozoite surface protein-1 (MSP-1). The other two monkeys served as non-immunized controls. PBMC were cultured for 24 h after stimulation with phytohaemagglutinin mitogen, MAP and MSP-1 antigens. Elevated expression of interleukin-6 (IL-6), IL-10, IL-12, tumour necrosis factor-alpha (TNF-alpha), TNF-beta and iNOS was seen in response to the MAP. Monkeys immunized with either P. falciparum MSP r190L or synthetic 190L peptides expressed predominantly the type-1 cytokines (IL-1 beta, IL-12, interferon-gamma, TNF-alpha, TNF-beta) characteristic of splenic, cell-mediated activity with macrophage activation and nitric oxide production.