Viable frozen lymphocytes displayed activity in blastogenesis assays that was indistinguishable from freshly prepared lymphoid cells. Similarly, cytotoxic activity of lymphocytes against melanoma target cells from melanoma patients was only slightly affected by the freezing procedure. Frozen lymphocytes provided a highly reproducible source of cells in these assays. The use of viable frozen peripheral blood lymphoid cells for the retrospective analysis of a cancer patient's immune response is described.