Blood cell differentiation and development is under the control of a number of genes, and mutations in one of these genes could result in an immunodefective phenotype. Mice of the CB17 scid/scid ("severe combined imunodeficient") strain are characterized by a mutation which affects early lymphoid differentiation and lack functional T and B lymphocytes. We investigated the serum level of murine immunoglobulins of CB17 scid/scid mice in comparison with the wild type CB17. Furthermore we performed FACS-analysis of the peripheral blood cells as well as cells from the spleen and thymus of these animals. Histological methods were used to study the morphology of the lymphoid tissues. In the sera of CB17 scid/scid mice murine immunoglobuline levels below 50 ng/ml were found. Mice with a level between this and the normal level (5-10 mg/ml) were called "leakys". Cytofluorometric as well as histological investigations revealed no differences between scid and leaky mice, which both appeared quite different than immunocompetent CB17 (wildtype). On the other hand animals with lymphomas show mononuclear infiltrations in the lymphoid tissues. Inflammatory reactions in scid are characterized by leukocytic infiltrations in the draining lymphnodes.