The immunotoxic effects and tissue distribution of different forms of methylmercury compounds were studied in rats. Methylmercury sulfide or methylmercury chloride was fed to rats at concentrations of 5 or 500 μg/L in drinking water for 8 wk. T‐cell lymphocyte proliferative response to phytohemagglutinin (PHA) and determination of tissue distribution of mercury by gas chromatography using electron capture were assayed. Four different forms of mercury compounds were employed: MeHgS−, (MeHg)2S, (MeHg)3S+, and MeHgCI. Results indicated that exposure to methylmercury significantly enhanced lymphocyte responsiveness in most of the exposed groups at the low concentration of 5 μg/L, with the highest proliferative response (fourfold increase) in the MeHgCl group. At 500 μg/L, a significant decrease in the lymphocyte proliferative response was observed in the (MeHg)3S+and MeHgCl groups; conversely, the MeHgS−and (MeHg)2S‐exposed animals had a modest increase of the lymphocyte proliferative response. The largest concentrations of all four mercury forms were detected in the kidney and spleen. The levels of mercury found in kidney, spleen, liver, brain, and testis were lower in the MeHgCl group than in those exposed to (MeHg)2S and (MeHg)3S+. These data indicate that the organ distribution of mercury and immune alteration may vary according to the chemical structure of the compound. This observation may have important implications in humans potentially exposed to low levels of methylmercury present in the environment, since the immune system plays an important regulatory role in the host‐defense mechanisms.