We report a method of developing a population of tumorocidal lymphocytes by culturing peripheral blood mononuclear cells (PBMC) with live isogenic colorectal tumour cells in a mixed cell culture. When the proportion of PBMC to tumour cells is 100:1, eradication of the tumour cell population results. The proportion of activated lymphocytes in culture increases with cytotoxic activity as demonstrated by the presence of the cytotoxic enzyme serine esterase in the lymphocyte granules.