The first purpose-built blood banks in the Turkish Republic and in the British colony of Cyprus were established in the 1950s. The life-saving medical services of these institutions were soon put to the test by outbreak of intercommunal violence in Cyprus and the ensuing hostilities against Greek Orthodox communities in Turkey. The scientific activities of the blood banks went well beyond the storage of blood for transfusions. The leading medical staff at these organizations examined the frequencies of ABO blood groups among their donor populations to evaluate their genetic relationships and racial classification. Based on scientific publications and archival documents from Turkey and the UK, this paper examines how the genetic research of the blood banks directly responded to the violence between Greek and Turkish Cypriots. At the end of the 1950s, the British prepared to end their colonial rule on the condition that Cyprus become independent rather than annexed to Greece (enosis). At this time, the British-employed leadership of the blood bank in Nicosia were sympathetic to the Turkish Cypriots and negatively inclined to the faction of Greek Cypriots determined to achieve enosis, whom they portrayed as irrational fanatics. After studying the blood of Cypriots from all backgrounds and across the island, they argued that not only was there no genetic difference between Greek and Turkish Cypriots, but that all Cypriots shared their ancestry with peoples from Anatolia, and had no biological connection to mainland Greece. Turkish hematologists working at the Istanbul blood banks translated these results as evidence that Cypriots were racially “Turks” and were inspired to carry out comparative studies in the 1960s that sampled blood from the Greek Orthodox community of Istanbul. On the basis of the blood group data, Turkish scientists argued that this community, like the Cypriots, shared its racial ancestry with Turks rather than mainland Greeks. Genetic markers in blood thus became the basis for instrumentalist political arguments about the illegitimacy of Greek territorial claims to Cyprus and parts of the Turkish Republic. The blood-based discourse of genetic kinship between Turks and Greek Orthodox communities did not reduce the intense political and social discrimination faced by Greek Orthodox people in Turkey. Instead, this paper argues, the Greek-Turkish case is only one early example of how the ongoing geneticization of identity and ancestry serves to deflect responsibility from the political forces that foment ethnic violence connected to nationalist territorial disputes.