A lymphokine activated killer (LAK) cell assay has been developed using a clonogenic microassay in agar-containing capillaries with KB tumor target cells. The assay avoids the problems of the commonly used 51Cr release assay and mimics physiological conditions more closely. The assay procedure has been optimized, resulting in the following conditions: LAK cells are generated by incubating nonadherent peripheral blood mononuclear cells from normal donors with 20 U/ml interleukin-2 for 3 days and cocultivated with 10(4) KB human squamous carcinoma cells/ml at 5:1, 10:1, and 20:1 effector:target ratios for 24 h. The cocultivation mixture is then seeded into agar-containing glass capillaries, allowing undamaged tumor cells to form colonies. The colony number is proportional to the number of tumor cells seeded. The present microassay requires up to 90% less cells and agent quantities compared with other clonogenic assays. It thus provides a useful tool to quantitate LAK cell activity and its alteration by immunomodulatory agents.