Clinical significance of white cell antibodies in febrile nonhemolytic transfusion reactions. Febrile nonhemolytic transfusion reactions (FNHTRs) are associated with white cell (WBC) antibodies. The purposes of this study were to determine the frequency of WBC antibodies, to associate the severity of reactions with antibody specificity, and to distinguish FNHTRs from infection and postoperative fever. By using the granulocyte indirect immunofluorescence test in conjunction with lymphocytotoxicity testing, it was found that 70 percent of FNHTRs in 24 patients involved WBC antibodies. The remaining 30 percent of apparent FNHTRs were associated with infections and postoperative fever. Granulocyte-specific antibodies were as prevalent as HLA antibodies and were associated with the severest reactions. Because FNHTRs occur with granulocyte-specific antibodies, HLA antibodies, and possible monocyte-specific antibodies (untested in this and other studies), these reactions could be grouped together as WBC-associated reactions. 