SUMMARY
Digestion of bovine erythrocytes (BE) with trypsin (Tr) or chymotrypsin (CTr) revealed Tr and CTr specific' receptor respectively. These receptor reacted with all the isologous, including the autologous, sera. The titres of agglutinability varied for the Tr‐digested BE from 1:2 to 1:64 and from zero to 1:128 for the CTr‐digested BE. However, the variation between the members of nomozygous twin pairs (MZ) in no case exceeded ±2 score units, the error limits. Agglutination of the CTr‐digested BE gave with the PHA lection very similar, (r= 0.964) results to that which was ontained with the isologous sera.The titre of agglutinins showed concordance in all the sera of MZ pairs when tested against the neuraminidas‐digested human erythrocytes and the Tr or CTr‐digested BE. Their variance due to differences varied between MZ pairs from 60.4 to 74.3%.