Chronic morphine treatment of rats for 2 and 4 weeks led to an increase in morphine-binding cells in the spleen, despite the general reduction of mononuclear cell content in the spleen and thymus. Simultaneously, serum antibodies to morphine (AbM) in haemagglutination titres 1:20 and higher appeared in 50% and 80% of animals, respectively. Animals with high titres of AbM had much weaker immune response to a thymus-dependent antigen (sheep red blood cells). In humans chronic opiate intoxication was accompanied by significant reduction of functional activity of T-lymphocytes, which was especially dramatic in addicts with high levels of antibodies to morphine: high incidence of infection was obvious in the latter group. Results suggest that high levels of antibodies to morphine serve as indicators, not only of chronic morphine intoxication, but of impaired immune reactivity, especially involving T-cell functional activity.