Polyagglutinable erythrocytes cause difficulty and delay in blood grouping and crossmatching. Abnormality of the erythrocyte membrane is the fundamental cause of some types of polyagglutinability. The altered chemical structure of the red cell membrane in various forms of polyagglutination isdescribed, with particular reference to the application of seed agglutinins in the classification of erythrocyte polyagglutinability. T-, Tn- and Cad-polyagglutination receive special attention and the ‘acquired B’ state is discussed. The haematological features of Tn-polyagglutination are also described.