Sera from 40 patients with febrile, nonhemolytic transfusion reactions were tested for the presence of alloantibodies using a number of techniques, including immunofluorescence tests on granulocytes, lymphocytes and platelets, a modified NIH lymphocytotoxicity test and the leukocyte agglutination test. Cells of at least 9 donors were used as target cells. Alloantibodies were detected in all sera. The frequency of the occurrence of antibodies was not much higher in sera obtained about 1 month after the transfusion reaction as in sera obtained within 4 days. Most of these antibodies were anti-HLA, but quite frequently platelet-specific antibodies were found, and sometimes these were the only antibodies detected. Granulocyte-specific antibodies were the least frequent. The nature of the antibodies was specified by their difference in reactivity with the cells of multiple donors, by applying panels of cells from typed donors and by absorption and elution experiments. It appeared that not only granulocyte-specific but also HLA- and perhaps platelet-specific antibodies may be responsible for a febrile transfusion reaction. We did not find that the occurrence of rigors, together with fever, was associated with particular serologic results.