The immunotoxicity of the glycol ether 2-methoxyethanol (ME) was evaluated in adult Fischer 344 rats using a variety of in vitro and in vivo immune function assays. In the first phase of this study, male rats were dosed by oral gavage with ME in water, at dosages ranging from 50 to 200 mg/kg/day, for 10 consecutive days. Decreases in thymus weights were observed at dosages of 50-200 mg/kg/day in the absence of decreased body weights. Lymphoproliferative (LP) responses to concanavalin A and phytohemagglutinin were reduced at 50-200 mg/kg/day while pokeweed mitogen and Salmonella typhimurium mitogen responses were reduced at 200 mg/kg/day. No alterations were observed in natural killer cell activity, mixed lymphocyte reaction, or cytotoxic T lymphocyte responses. The frequency of W3/25-positive splenocytes was reduced in rats dosed at 200 mg/kg/day. Interleukin-2 production was reduced in splenocytes from rats exposed to all dosages of ME. The plaque-forming cell (PFC) response to sheep red blood cells was enhanced in rats dosed at 50 mg/kg/day. However, the PFC response to trinitrophenyl-lipopolysaccharide (TNP-LPS) was suppressed at all dosages. Similarly, the PFC response to TNP-LPS was suppressed in adult female rats dosed with ME. A reduction in the expulsion of adult worms was observed in rats dosed at 200 mg/kg/day that were infected with Trichinella spiralis. A number of male reproductive parameters were also evaluated in rats dosed with ME over 10 days. A significant reduction in testicular weight was observed in rats dosed at 200 mg/kg/day. In the second phase of this study, the PFC response to TNP-LPS was employed to assess the role that metabolism of ME to 2-methoxyacetic acid (MAA) plays in the immunotoxicity of this glycol ether. Ten-day oral dosing with MAA resulted in the inhibition of the PFC response to TNP-LPS at dosages of 50-200 mg/kg/day. Concomitant exposure of rats to ME and the alcohol dehydrogenase inhibitor 4-methylpyrazole blocked ME-induced suppression of this PFC response. Attempts to ameliorate ME-induced suppression of the PFC response with serine, which has been shown to reverse ME-induced developmental and reproductive toxicity, were unsuccessful. These results suggest that the immune system may be more sensitive than the reproductive system to the toxic effects of ME. Furthermore, it appears that MAA is the proximate toxicant for ME-induced alterations in the immune system, as has been demonstrated for ME-induced reproductive and developmental toxicity.