Diabetogenic Coxsackievirus B4 infection may trigger autoimmune islet loss in diabetes-susceptible mice, resulting in hyperglycemia in nearly 90% of the animals at 6–8 weeks postinfection (p.i.). To ascertain whether changes in lymphocyte repertoire following infection could predispose these animals to diabetes, alterations in their thymic, splenic, and peripheral lymphocytes were analyzed. Additionally, lymphocyte changes were correlated with the virus load in these tissues and with lymphocyte migration to the inflammatory pancreas. Splenic B lymphocytes more than doubled at 72 hr p.i. and then continuously decreased by 16% of the noninfected controls at 8 weeks p.i. T lymphocytes (CD4+ + CD8+) decreased by about 50% at 72 hr and then increased to the control level by 8 weeks p.i.; CD8 subset continuously decreased by 40% of the control at 8 weeks, resulting in a 67% increase in CD4+/CD8+ ratio. Macrophages and CD5+B subset increased at 72 hr and then dipped by 93% and 84%, respectively, at 8 weeks. In contrast, peripheral B lymphocytes increased by 74% and T lymphocytes decreased by 11% a t 8 weeks p.i. Macrophages increased by twofold at 72 hr and then dipped slightly (6%) at 8 weeks, whereas CD5+B subset increased by 245%. Most prominent thymic T lymphocyte alteration was reflected by about 150% increase in CD4− CD8− cells at 8 weeks p.i. The peak viremia occurred at 72 hr p.i., with highest and lowest virus in the spleen and thymus, respectively. The thymus cleared virus by 3 days, the other tissues by 7 days. lnsulitis and acinar necrosis followed infection; infiltrating lymphocytes were mostly CD4−. Virus-induced abnormal lymphocyte maturation may contribute to the development of insulitis and hyperglycemia. © 1992 Wiley-Liss, Inc.