Diabetic patients with the disease duration less than 1 year (group 1 eight patients with diabetes mellitus type I and group 2 seven patients with diabetes mellitus type II) and 8 healthy donors were examined for subpopulations of lymphocytes (CD 3-, CD 4-, CD 8-, CD 20-positive cells, CD4/CD8), expression of activation markers on peripheral blood mononuclears investigated with monoclonal antibodies of BMA and OKT series, and serum neopterin concentration. Group I patients had low CD4/CD8 and increased number of CD 8 cells, 1a/DR-positive lymphocytes (28.6 +/- 7%), OKT9-positive lymphocytes (8.0 +/- 4.7%), activated neopterin synthesis registered neither in group 2 patients nor donors. The number of CD 3 and CD 4 cells was similar in the diabetics and donors. B-lymphocyte level in group 1 patients was on the decrease. Unbalance in lymphocyte subpopulations, increased expression of activation markers and of serum neopterin can be noted in viral infection reflecting impairment of immunoregulating mechanisms in diabetes mellitus type I.