Humans are exposed to many commercial and consumer products that contain siloxanes. We investigated the immunological effects of a two-week oral exposure to octamethyicyclotetrasiloxane (Da). Normal volunteers ingested either 12 mg of 04 in com oil or corn oil alone in a double-blind, placebo-controlled, crossover study design. Immunological assays were chosen to screen for an immunotoxic or adjuvant effect. Assays for immunatoxicity included enumeration of peripheral lymphocyte subsets and functional assays using peripheral blood mononuclear cells. Because in humans there is no direct test for adjuvant effect of siloxane exposure, we analyzed pro-inflammatory cytokines and acute phase reactants in peripheral blood as surrogate markers for adjuvancy. Blood was obtained prior to exposure, and after 7 and 14 days of exposure. In these controlled human exposures, no immunotoxic or pro-inflammatonfladjuvant effect of D4 ingestion was found.