Immunosuppression of humoral and cellular responses following chronic oral exposure to 1, 5, 10, and 20 ppm N‐nitrosodimethylamine (NDMA) was examined in CD‐1 mice. Monitoring of cumulative mortality and the incidence of peritoneal ascites in animals showed an NDMA dose‐related mortality and hepatotoxicity. No visible changes in immunological parameters were noted at the 1 ppm NDMA dose. Immunosuppression of immunoglobulin M (IgM) antibody response by NDMA to sheep red blood cells (SRBC) was time‐related, dose‐related, and could be reversed within 30 d by removal of the chemical from the drinking water. Cellular immune response, monitored by allogeneic stimulation of cells in mixed lymphocyte reaction (MLR), was markedly suppressed by 10 and 20 ppm NDMA. Thus, chronic exposure to NDMA, except for the low‐hepatotoxic doses of nitrosamine, resulted in a marked and persistent immunosuppression of cellular and humoral responses in CD‐1 mice. In conclusion, chronic exposure to the hepatotoxic (ascite‐inducing) doses of NDMA suppressed humoral and cellular immunity. The persistant immunosuppression could be reversed after the removal of NDMA from the drinking water. Although no direct NDMA‐related cancer was reported in humans, our data point to a potential epigenetic carcinogenicity of nitrosamines due to chronic immunosuppression.