This study was undertaken to document the incidence of immediate, nonhemolytic transfusion reactions and to identify a technique or set of techniques that would best identify the different causes of these reactions. A variety of tests were employed to detect lymphocyte, granulocyte, platelet and anti-IgA antibodies. During this study 26,318 units of blood components were transfused on 5,030 occasions. 191 immediate, nonhemolytic reactions were experienced giving an incidence per unit of 0.73%. Blood specimens from 101 of these patients were investigated along with serum from 57 patients who showed no reaction to transfusion as controls. We show that standard B cell lymphocytotoxicity testing is the technique with which most antibodies can be detected (64% of reactors positive vs. 30% of controls, p< 0.001). Additional tests did not significantly increase the level of antibody detection.