In our examinations the cytochemical characters of the peritoneal macrophages of immunosuppressed mice (Is-PM) were studied. It was stated that the acid phosphatase activity of their lysosomal system decreased in comparison to control cells (N-PM). In Is-PM cells the polyanion content of the cell-coat and the expression of their ConA receptors decreased, while the glycos- and galactosaminoglycan (GAG) content showed a decrease on the surface of every cell of the population. On the first day following carrageenan treatment, acid phosphatase activity of the cells was very low, the GAG content and the conA receptors of the cell-coat disappeared. The characteristic lysosomal alterations could still be observed on the 6th day after treatment. On the basis of our observations treatment with carrageenan impaired not only the lysosomal system of macrophages, but the cell surface too. These cytochemical changes may explain the dramatic decrease in macrophage function observed in carrageenan treatment.