After the leukemic cell lines were treated with monoclonal antibodies (McAbs) and interferon (IFN-alpha), the changes of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) levels in the corresponding leukemic cell lines were measured by radioimmunoassay. The results showed that when the ratio of antigen to antibody was 80 to 1, the cAMP levels in the leukemic cell lines were obviously higher than those in the controls while the cGMP levels were obviously lower after being treated with the corresponding McAbs for 16-24 h (P < 0.001). The average level of intracellular cAMP was remarkably increased and that of cGMP underwent no significant changes in the leukemic cell lines after treatment with IFN-alpha.