Using increased volumens of serum [in the Multiple-Dosage (MD) or a High Dosage (HD) test] with a sensitive manual reading technique all sera of 37 donors of blood group A2 showed irregular anti-A1(alpha1) antibodies. With the same technique, up to 1/3 of the sera of 417 healthy blood donors agglutinated red cells of a selected antigen pattern at room temperature; at 4 degrees C the ratio of positive reacting sera was much higher. The specificity of the antibodies, as indicated by the antigramme type, could be confirmed in the most cases by reacting with a series of known positive and negative reference red cells. The eluates of some of the sera yielded the same antigramme pattern as the native sera did. Some of the antibodies showed a specificity corresponding to a red cell property of the respective blood donor; but not in each case the antibodies were capable to agglutinate the donor's cells. If "autoagglutinins" were strong, frequently they revealed a character of panagglutinability, particular in the cold. With the antiglobulin test, which followed the NaCl-MD procedure, the number of positive reactions increased; on the other hand, there were some of the previously positive agglutination tests, which became negative during the course of the AHG step.