Measurement of anti-A and anti-B haemagglutinating alloantibody titres was performed on serum samples from both pedigree (n = 61) and non-pedigree (n = 43) cats previously typed using a commercial blood typing kit. All 40 type B cats had anti-A antibody with titres ranging from 4 to 1600. Among the 61 type A sera tested, an anti-B agglutination titre of greater than 2 was recorded in 16-4 per cent. There was no significant association between serum alloantibody titre and cat breed or gender (P>0·05).