After observing the increased lymphokine-activated killer (LAK) cell sensitivity of the multidrug resistant (MDR) P-gp(+) small cell lung carcinoma line H69/LX4, but not the multidrug resistant-associated protein(+), P-gp(-) and glutathione-S-transferase pi (+) variant H69/AR compared to the parental line H69 [15], we chose to investigate LAK cell susceptibility of mouse and human multi-drug resistance 1 (MDR1) gene transfected MDR cell lines. The gene discussed is PGP; the disease is small cell lung carcinoma.