The study was performed in 39 children with newly diagnosed insulin-dependent diabetes mellitus. The %age number of CD19+, CD5+/CD20+ B lymphocytes and CD3+, CD4+, CD8+, CD4+/DR+, CD8+/DR+ T lymphocytes subpopulations were investigated in the peripheral blood. Marked, statistically significant increase (p<0.0001) in CD5+ B lymphocytes was revealed in 77% of patients as compared to the healthy control. The elevated number of CD5+ B lymphocytes correlated with presence of activated lymphocytes T (CD4+/DR+ and CD8+/DR+). The total number of CD19+, CCD3+, CD4+, CD8+ lymphocytes was on comparable level in both groups.