Spatial autocorrelation analyses of ABO serotypes were carried out for three medieval Hungarian cemeteries in an effort to detect familial structure. Different ethnic groups buried in separate areas dominated the spatial pattern in two cemeteries, leaving insufficient power to test for familial patterns. In a third, ethnically homogeneous cemetery, no evidence of familial structure was found. A simulation showed that familial structure could readily be detected by the methods applied when it exists in an ethnically homogeneous population. The spatial autocorrelation methods employed would have detected the ethnic diversity in the two cemeteries containing graves from different populations, even in the absence of archaeological information to that effect. -- AATA