The ABO blood types of the dead are of intense interest to forensic serologists and other medicolegal investigators because they may allow investigators to limit significantly the number of antemortem dental or fingerprint records that are needed for identification. Physical anthropologists study the ABO grouping of human remains because such grouping provides direct information concerning the genetic makeup of ancient human populations. The author reviews the methodology used at present to group bones and other tissue samples and examines the serious problems presented by postmortem microbial activity. Recent research indicates that microorganisms may alter or eliminate blood group activity; microbial antigens with A and B serological activity may also be detected in putrefying remains.