The direct antiglobulin test (DAT) was positivein 134 (36.1%) of 371 anemic dogs with internal diseases. Four principal types of reaction were recognized: IgG alone in 15 (11.2%), IgG + C' in 41 (30.6%), C' alone in 74 (55.2%) and IgM + C' in 2 (1.5%). Rarely, IgM and/or IgA reactions occurred in association with strong IgG + C' reactions. In 2 (1.5%) DAT-positive dogs the type of reaction was not clear. One or more symptoms of hemolysis, such as hemoglobinemia, indirect type hyperbilirubinemia, increased red cell osmotic fragility, and increased fecal urobilinogen excretion, were demonstrated in 84 DAT-positive dogs. These consisted of 10 of 15 dogs with IgG type DAT, 36 of 41 dogs with IgG + C' type DAT, 36 of 74 dogs with C' type DAT and 2 of 2 dogs with IgM + C' type DAT. Most dogs with IgG + C' type reactions had severe hemolysis, whereas "primary" or "associated" diseases were recognized in only 26 of 56 cases. IgG type incomplete warm antibody, reacting with pooled donor cells, was demonstrated in red cell eluates in each of 3 dogs with IgG type DAT and in 6 of 7 dogs with IgG + C' type reactions. This indicates that dogs with IgG or IgG + C' reactions usually have autoimmune hemolytic anemia. In dogs with C' type DAT, indications of hemolysis were frequently minimal or absent. Symptoms almost always indicated some "primary" disorder. Diagnoses mainly included infections, inflammatory and neoplastic (especially myelo- and lymphoproliferative)diseases. In only 7 (9.5%) of 74 dogs with C' type DAT no diagnosis other than (transient peracute) hemolytic anemia was made. The results of tests for antibodies in the serum and red cell eluates were always negative in dogs with C' type DAT. In one dog with hemolytic anemia and C' + IgM type DAT, there was a high titer of IgM cold agglutinins in the serum and in heat eluates. It is concluded that a positive DAT with anti-IgG antiserum is a strong indication of autoimmune hemolytic anemia but that a reaction of the C' alone type is a rather common phenomenon in canine internal diseases which is seldom associated with serious hemolysis.