At the present time, because of lack of investigation and/or conflicting reports in the literature, it is not known whether the number of T cells decreases with increasing age or whether T cells in humans become functionally less effective with age. The objective of this project is to determine (a) the nature of the decline and (b) the mechanisms responsible. Both T and B cell functions will be assayed by immunofluorescence, SRBC rosettes, response to mitogens and antigens, and skin tests. It was found that in patients with Hodgkin's disease "killer" T cells cytolyse autologous neoplastic Reed-Sternberg cells in the following sequential manner: stage 1, T cells affix the tips of their microvilli onto target cells; stage 2, T cells subject target cell membranes to shearing and tearing forces which produce gaps and holes; stage 3, target cells lyse and T cells "crawl" away.