The immunotoxicity resulting from subchronic inhalation exposure to isobutyl-nitrite (542563) was investigated in mice. BALB/c-mice underwent inhalation exposure to isobutyl-nitrite in concentrations of either 50 or 300 parts per million 6.5 hours a day, 5 days a week, for up to 18 weeks. During exposure weeks 7, 13, and 18, 20 animals from each exposure group were sacrificed, blood samples were collected for hematological testing, and the spleens were aseptically removed. Antibody producing cells were enumerated by slide plaque assay for animals immunized with sheep red blood cells while still in the exposure chamber. Lymphocyte proliferative response to phytohemagglutinin, concanavalin-A, pokeweed mitogen, and lipopolysaccharide was tested at several concentrations. Additional animals were immunized with a complete adjuvant 21 days prior to sacrifice and were subsequently tested for delayed hypersensitive responses to a purified protein derivative via radiometric skin tests. The relative number of T-cells and T-cell subsets present among the splenic lymphocytes taken from both test and control mice was determined. No discernible immunotoxic effects were observed in animals subjected to isobutyl-nitrite inhalation during any of the assays performed. The authors conclude that isobutyl-nitrite exerts no discernible detrimental influence on the immune systems of mice at the doses tested.