Previous studies indicated that lymphoid tissues, especially the thymus, became atrophied and the proportion of sheep-erythrocyte-rosette-forming cells declined in individuals with protein energy malnutrition (PEM). Nevertheless, the mechanism of immune deficiency in malnutrition remains unclear. We hypothesized that the low proportion of rosette forming cells and dysfunction of lymphocytes may be caused by CD2 receptors that work improperly. 20 Wistar-albino rats received a diet containing 5.5% protein for 3 weeks. A control group was fed a normal diet containing 24% protein. By the end of 3 weeks there was a 18% weight loss, and serum total protein had decreased significantly, in the protein deficient group. Lymphocyte sub-populations including CD3+, CD19+, CD4+, CD8+ and CD2+ cells were investigated by immunofluorescence. Immunoglobulin G (IgG), IgM and IgA levels were measured by radial-immunodiffusion plaques. The proportion of CD3+ cells did not change while those of CD19+ and CD8+ cells increased significantly, with significant declines in CD4+ and CD2+, in peripheral blood. There was a moderate correlation (r = 0.52) between the declines in IgG and IgM levels, and that in the CD4+ cell proportion as well as the CD4/CD8 proportions. We believe that PEM causes a decrease in the number and/or function of lymphocyte subpopulations. As a result cell activation and antibody production are impaired. Med Sci Res 26:763-766 (C) 1998 Lippincott Williams & Wilkins.