Human lymphocytes (purified from defibrinated venous blood using gelatin sedimentation, nylon column incubation, and Tris NH⁴Cl lysis of contaminating erythrocytes) were tested for cytotoxicity (CTX) against T24 cells derived from a carcinoma of the bladder and against cell cultures derived from other tissues. The CTX was assessed in a modified microcytotoxicity test (MCT) where loss of ³H‐proline prelabelled monolayer target cells from the surface of micro‐wells indicates destruction of these cells. The number of target cells that remained attached after incubation with lymphocytes (measured as residual ³H‐counts) was compared to the number that remained attached after incubation with control lymphocytes. The results were classified into three categories: (1) specific CTX (destruction only of T24 cells); (2) non‐specific CTX (destruction ofT24 cells as well as other target cells); and (3) negative (no destruction of any type of target cell). Specific CTX for T24 cells was seen more often with lymphocytes from bladder cancer patients (10/27) than from other patients and normal individuals (2/49) (p <0.001) which suggests that this is a disease‐related phenomenon. By contrast, non‐specific CTX occurred with equal frequency amopg bladder patients (6/27) and other patients (9/36), although less frequently among normal individuals (0/13). In addition, lymphocytes that showed specific CTX for T24 cells were found more often in patients with early or superficial bladder cancer (8/15) and less often in patients with advanced bladder cancer (2/12). These results are in agreement with those obtained by others who used the standard MCT that measures target‐cell destruction and influences on target‐cell proliferation.