Viable cultured oat-cell carcinoma cells were used to immunize a goat. The resulting antiserum contained high titres of anti-normal activity and antibodies to CEA. It was also shown, by indirect immunofluorescence, using fluorescein-conjugated rabbit anti-goat Ig, to localize at high titres on the surface membranes of human lung cancer cells of 4 different histological types. Booster immunizations produced a maximum secondary response one week after 2 weekly injections. The course of each immunization has been monitored for activity against normal human tissues, and the final sera have been absorbed with human spleen cells to remove anti-normal activity. Cross-reactivity with the lung-cancer-cell panel and antibodies to CEA persisted in high titre after absorption of anti-normal antibodies, and were present in the ammonium-sulphate-precipitated globulin fraction. The cells used for immunization did not produce detectable amounts of CEA in culture, and were not known to contain CEA prior to this experiment. Removal of anti-CEA antibodies by absorption with purified CEA has not reduced the cross-reactivity of the absorbed antiserum with the panel of human lung-cancer cells.