The molecular aspects of T-cell activation and subsequent progression through the cell cycle are investigated using the murine model to establish a basis for the immunodeficiencies associated with advancing age. Progression through and exit from the G1 phase of the cell cycle is limiting in a significant portion of the T-cells derived from old mice. A subfamily of the cyclin-dependent kinases (cdk) represented by cdk4 and cdk6 are though to play a significant role during G1. Prior to initiating studies with old mice, T-cells derived from young animals were used to characterize cdk4 and cdk6 during the G1 phase of the cell cycle following polyclonal activation with immobilized anti-CD3. These studies have shown that: 1) Specific mRNA for cdk4 and cdk6 are detected 5 hr after activation and maximum levels for cdk4 are attained at 10 hr and 20 hr for cdk6. 2) Protein levels of both kinases were first detected at 5 hr and were significantly increased by 20 hr. 3) D cyclins were consistently found to be associated with cdk4 and cdk6. 4) Although both kinases phosphorylated retinoblastoma protein and E1A binding protein the kinetics of the reaction were different. 5) The availability of cyclin D2 and D3 during the G1 phase of the cell cycle may account for the timing of the kinase reaction. Studies will now be undertaken using T-cells derived from old mice to determine the role of cdk4 and cdk6 and the D cyclins in the failure of activated T-cells to progress through and exit G1.