Serum immunoglobulin and natural antibody concentrations to environmental antigens have been determined in 26 hairy cell leukaemia (HCL) patients. The serum IgG, IgA and IgM levels as well as the concentration of antibodies to the majority of intestinal bacteria were found to be normal in both groups of HCL patients. In contrast to the data on the impaired humoral immunity of chronic lymphocytic leukaemia (CLL) patients, our results show that in HCL the studied aspects of humoral immunity are normal. Serum IgE levels were found to be significantly decreased in HCL when all patients were taken as a group. The difference was due to the decreased serum IgE concentration of splenectomized HCL patients, while IgE levels of untreated HCL patients did not differ statistically from that of the healthy controls.