The lymphocytes of patients with inoperable bronchus carcinoma were investigated for their in vitro reactivity toward tumor-associated antigens. The leucocyte-migration-inhibition-test (LMIT) was applied using formalinised tumor cells and tumor cell hybrid cell lines. 34 patients with inoperable suqamous cell carcinoma of the lung and 21 healthy controls were tested for their leucocyte sensitisation to these cells. 55 percent of the patients with lung cancer showed a specific LMI toward the E-14 cell line, (an established cell line from a human squamous cell carcinoma of the lung), compared to only 9 percent of the controls. There was about the same percentage of specific reactions of the lymphocytes to three different hybrid cell lines which have only a few human chromosomes left over. There was no reactivity to the parental hamster fibroblasts. It is assumed that the hybrid cells carry the same tumor-associated antigens on their cell surfaces as the E-14 tumor cells.