In vitro cell kinetic studies of 27 surgically removed human lung cancers were performed by the immunohistochemical procedure using monoclonal antibody to bromodeoxyuridine (BrdU). BrdU was incorporated under hyperbaric oxygenation, and labeled cells were found to be evenly distributed in the tissue blocks. A comparison of the in vitro and in vivo labeling index (L.I.) in human tumors transplanted to nude mice showed good correlation. Surgically resected adenocarcinoma of the human lung had a significantly lower L.I. than squamous cell carcinoma, the mean values being 6.0% and 17.2%, respectively. In adenocarcinoma, L.I. values disclosed a trend toward a higher L.I. in poorly differentiated carcinoma.