The parameters of cellular and humoral immune responsiveness were studied during the first days of morbilli rush in the group of 34 young adult patients. The analysis of cell-mediated immune response included T cell relative and absolute numbers, mitogen-induced T cell proliferative response, as well as the relative and absolute numbers of mononuclear phagocytes. The tested parameters of humoral immune response included B cell relative and absolute numbers, serum concentrations of immunoglobulins IgG, IgA and IgM, and serum level of immune complexes (IC). The reduction of total leukocyte number, T cell and B cell relative and absolute numbers, as well as diminished mitogen-induced T cell proliferative response, associated with the elevation of IgM and IC serum levels were found in the majority of analyzed patients. Moreover, the subsequent analysis revealed positive correlations between T cell percentage and T cell proliferative response, as well as between serum concentrations of IgM and IC. These data confirmed the presence of acute morbilli infection-caused disturbances of patients' immunocompetence.