The technique of human tumor cell line transplantation in immunodeficient mice is used worldwide as a model for cancer research. In accordance with international recommendations, animals used in biomedical research should be free of microorganisms which can interfere in experimental results; including Pasteurella pneumotropica. The object of this study was to evaluate the interference produced by P. pneumotropica in the human adenocarcinoma cell line A549 transplanted in N:NIH(S)-nu mice. A total of 40 mice divided into 4 groups of 10 animals each was used to perform this study. Group 1: inoculated with the cell line; group 2, with the bacteria; group 3, with the cell line and the bacteria; group 4, as control with no inoculations. Significant differences were observed in tumor growth in groups 1 and 3, infected and not infected with P. pneumotropica. Although this microorganism is non lethal and only opportunistic, the infected animals are to be considered not suitable to be transplanted with the tumor cell line A549 for experimental studies since these bacteria interfere with tumor growth. However, the fact that a growing tumor regresses in the presence of the bacteria is an interesting observation which deserves further exploration in order to elucidate the mechanism involved.