Dimethylglycine (DMG), a tertiary amino acid, has had wide acceptance as a nonfuel nutrient; presumably it enhances oxygen utilization by tissue and complexes free radicals. Its potential as an immunoadjuvant has also been suggested by a study of an analog of DMG, calcium pangamate. A double-blind study in 20 human volunteers showed a fourfold increase in antibody response to pneumococcal vaccine in those receiving DMG orally as compared with controls (P less than 0.01). Production of leukocyte inhibitory factor in response to concanavalin A was similar in the two groups, but those taking DMG tablets had a significantly highr mean response of leukocyte inhibition factor to streptokinase-streptodornase (P less than 0.001). The in vitro responses of lymphocytes from patients with diabetes and those with sickle cell disease to phytohemagglutinin, convanavalin A, and pokeweed mitogen were increased almost threefold after addition of DMA. These results suggest that DMG enhances both humoral and cell-mediated immune responses in humans.