A review of the earliest publications and letters of the scientists who have received credit for the first description of red cells illustrates the philosophical and practical difficulties of assigning priorities for discoveries. A knowledge of the scientific and cultural ambience of the day, of reports of scientific contemporaries or predecessors, both partisans and adversaries, and other background information illustrates how the consensus in crediting the discovery originated. However, the broader question remains: what are the proper criteria for assigning priority of discovery, yesterday, today and tomorrow?