An alloantibody to a high-incidence antigen, associated with multiple other alloantibodies, made it impossible to supply antigen-negative red blood cells (RBCs) for a chronically transfused sickle cell anemia patient. Anti-Cra,-E,K,-S, -Fya, -Fyb, as well as anti-M reactive at 37 degrees C and in the antiglobulin phase of testing, were identified in the patient's serum. An extensive search of rare donor files at the American Red Cross and at the American ASsociation of Blood Banks (AABB) failed to identify Cr(a-),M-,E-,K-,S-, Fy(a-b-) donors. Various studies were performed to predict the clinical significance of the anti-Cra and anti-M. Results of 51chromium survival studies showed 91.8 percent survival at 10 minutes and 87.2 percent survival at 60 minutes with Cr(a +),M-, K-,S-,Fy(a-b-) donors. Various studies were performed to predict the clinical significance of the anti-Cra and anti-M. Results of 51chromium survival studies showed 91.8 percent survival at 10 minutes and 87.2 percent survival at 60 minutes with Cr(a +),M-,E -,K-,S-,Fy(a-b-) red cells, suggesting that immediate destruction of transfused CrCa+) red cells would he unlikely. However, further analysis revealed diminished long-term survival of the donor's red cells with only 60.1 percent recovery at six days (T 1/2 = 12 days) and 10.8 percent at 14 days (T 1/2 = 4.5 days). A monocyte- monolayer assay (MMA) indicated that both the anti-Cra (5.9%) and anti-M (18%) would probably be clinically significant (normal value 0-3%). Mass screening continues at several blood centers for Cr(a-),M-, E-,K-,S-,Fy(a-b-) donors. However, if no suitable donors are found, the results of the 51chromium survival studies and the MMA support the decision to transfuse this patient with Cr(a+),M-,F(a- b-),S-,K- ,E- red cells, if necessary.