Scanning electron micrography of red cells agglutinated by various blood group antibodies has shown that the agglutinates formed are of two completely different morphologic types. Type I, obtained with anti-A sera (IgM and IgG) as well as with anti-A(1) lectin, consists of interlaced, crenated and spiculated cells, while type II, obtained with anti-D sera (complete and incomplete) consists of densely but randomly packed smooth, discoid cells. Papain treatment results in indented, spheroid cells with a much corroded surface. Coombs-type antiglobulin agglutinates consist of spheroid cells, the coating of which tends to form a continuous envelope, interconnecting several cells. The influence of heterogeneic and of viral agglutinins is also illustrated.