Twenty mice were acutely exposed to 1.5% halothane anaesthesia for 4 h. The mice were sequentially killed during the first 3 days after halothane exposure. Another 14 mice were chronically exposed to 0.25% halothane for 1 h daily, four times weekly for 3 months. The mice were sequentially killed every week. Acute exposure to halothane anaesthesia resulted in an increase in lymphocyte count per spleen, a decrease in serum IgG concentrations (Day 2), a reduction in spontaneous 3H-thymidine lymphocytic uptake (Day 1), and an increase in concanavalin-A-stimulated uptake. All immunometric assays returned to control levels on Day 3 after halothane exposure. Chronic non-anaesthetic concentrations of halothane exposure produced a decrease in serum IgG concentrations and an increase in spontaneous and stimulated lymphocyte 3H-thymidine uptake. It is concluded that acute exposure of mice to anaesthetic halothane results in a transient depression of the immune response, while chronic non-anaesthetic concentrations produce a differential effect on the two moieties of the immune system, a depressed humoral response and overactive cellular response.