Juvenile-onset diabetics (JOD) have a significantly low peripheral T-lymphocyte count. In vitro exposure of diabetic lymphocyte cultures to insulin causes a significant T-cells count increase and thereafter no significant difference is detectable among JOD, maturity-onset diabetics (MOD) and normal subjects (NS) T-cells counts. This finding supports the hypothesis that the T-lymphocyte depressed function present in JOD is a secondary phenomenon due to in vivo insulin deficiency. Possible mechanisms of insulin action in restoring E-rosette function are discussed.