The red blood cells of Mr. R. W., an 86‐year‐old type O patient with Proteus mirabilis wound infection, anemia, and thrombocytopenia were found to be polyagglutinable due to Tn‐activation, to be agglutinated by and to absorb anti‐A, and to have a reduced sialic acid level probably as a result of enzyme modification in vivo. The reaction with anti‐A was inhibited by blood group A substance. It is possible that both Tn‐activation and acquisition of A‐like antigen were changes induced by Proteus infection but, if so, the changes persisted after the bacterial infection was cured. The red blood cell modification in this patient has features in common with others in whom T‐ or Tn‐activation, reduced sialic acid levels, or acquired B antigens have been found. Various abnormalities of the red blood cell membrane are discussed and clinical, serological, and physicochemical findings are compared.

T‐ and Tn‐activation are known to result in polyagglutinability of red blood cells. Unlike polyagglutinability associated with the presence of the Cad antigen, there is no association in these two states with the antigen Sda. Alteration or destruction of the T‐receptor on T‐activated cells by the proteases fidn and papain is strictly dependent on the strength of the protease solution used. There is no correlation of amounts of anti‐T and anti‐Tn in normal sera.