Previous reports on the clinical significance of anti-Csa(Cost-Stirling) have presented conflicting data. We report our findings, over an 8-month period, of a patient whose serum contained anti-Csa and anti-Fya. Nineteen donor units of ABO and Rh-matched, Fya-negative red cells, which were crossmatch incompatible, were transfused with no clinical, serological, or biochemical evidence of a hemolytic transfusion reaction.