The purpose of this study was to evaluate immunological status in dogs with cancers at different stages, in comparison with normal dogs. The population of canine peripheral blood lymphocytes (cPBL), lymphocyte phenotypes, interleukin (IL)-6 activity and alpha 1-acid glycoprotein (alpha(1)-AG) level were analyzed. The tumor-bearing dogs had higher numbers of leukocytes than normal dogs, the count being higher in dogs with more advanced tumors. In the tumor-bearing dogs, differential leukocyte counts revealed higher percentages of inflammatory cells such as neutrophils, acidophils and monocytes, and lower numbers of CD4(+)T cells, than in normal dogs, the lymphocyte counts becoming much lower with tumor progression. In the tumor-bearing dogs, the CD8(+)T cell count at the early tumor stage was similar to that in normal dogs, but decreased with tumor progression, possibly reflecting the development of humoral immunity (Th2). Plasma IL-6 and TGF-beta activities were high in the tumor-bearing dogs. The plasma alpha(1)-AG concentration was also significantly high in the tumor-bearing dogs. Our findings suggest that assay of IL-6, TGF-beta and alpha(1)-AG may be very useful for prognostication in dogs with cancer, and that anti-tumor immunity is potently suppressed in such dogs.