Standardized exposure conditions with Kentucky reference 2R1 cigarettes were used to expose 2,053 (C57BL/Cum x C3H/AnfCum)F1 female mice (nose only) to fresh, whole cigarette smoke. In addition, 1,014 mice were sham-exposed, and 449 mice were held as shelf controls. The protocol entailed exposing mice to smoke (or sham-exposure) on a daily basis, 5 days/week, for 110 weeks and observing remaining mice until death. A large number of animals was used so that the smoke generation and animal-holding systems could be tested and evaluated and yet provide significant numbers of animals for exposure to cigarette smoke for a major portion of their lifetime. Deposition of smoke particulates was estimated to be about 125-200 micrograms total particulate matter/lung/day. The only lung cancers observed were diagnosed as alveolar adenocarcinomas (AAC). A total of 19 of 978 smoke-exposed mice and 7 of 651 sham-exposed mice were observed with AAC. The difference between the smoke-and sham-exposed groups was not statistically significant at P less than or equal to .05, but the data suggested that the tumors occurred with a shorter latency in the smoke-exposed group (P = .10). The data were analyzed by various methods, including analysis of subsets of the population of animals. A significant increase in the incidence of lung cancer was observed in one subset; however, this difference was not found in the population as a whole or as a result of any other analyses. Under these exposure conditions, 2R1 cigarette smoke would seem to have weak carcinogenic activity in mouse lung tissue. Other changes associated with smoke exposure were increased incidence of pigmented alveolar macrophage accumulation, otitis media, and head and neck fibrosarcomas. However, the incidence of nephritis, hematopoietic cancers (e.g., leukemias, lymphosarcomas, and reticulum cell sarcomas), and pulmonary congestion was significantly higher in the sham-exposed animals.