Peripheral blood lymphocyte subsets were determined in former workers of the Okunojima Poison Gas Factory (poison gas workers) having a high incidence of lung cancer, and the effect of administration of Nocardia rubra cell wall skeleton (N-CWS) was studied. In poison gas workers not receiving N-CWS, both the percentage and absolute number of Leu-2a+ cells were significantly increased as compared with normal controls, while both the percentage and absolute number of Leu-7+ cells as well as the Leu-3a/Leu-2a ratio were significantly decreased. In poison gas workers receiving N-CWS, the percentage of Leu-2a+ cells was significantly decreased when compared with poison gas workers not receiving N-CWS, while the percentage of Leu-1+ cells and both the percentage and absolute number of Leu-7+ cells as well as the Leu-3a/Leu-2a ratio were significantly increased. When N-CWS was administered to poison gas workers, the percentage of Leu-2a+ cells significantly decreased, with a minimum two weeks after administration, while the percentage of Leu-3a+ and the Leu-3a/Leu-2a ratio significantly increased and the percentages of Leu-1+ and Leu-7+ cells also increased. Furthermore, the absolute number of Leu-3a+ cells increased markedly with a peak two weeks after administration. The absolute numbers of both Leu-1+ cells and Leu-7+ cells as well as total lymphocytes also significantly increased, but that of Leu-2a+ cells showed hardly any change. It is considered that a single administration of N-CWS transiently corrects the abnormality of lymphocyte subsets, and that repeated administration of N-CWS once every three months may maintain the lymphocyte subsets of poison gas workers at normal levels.