A new immunoenzymatic assay for the detection of serum-free and platelet-associated (PA) antibodies is described. This method evidences the platelet bound IgG employing the biotin-avidin system. Following a common sensitization step, the PA-IgG can be directly detected on the cell membranes, and their amount can be evaluated by an anti-globulin consumption assay. In comparison with the platelet suspension immunofluorescence test (PSIFT), the ELISA assay is a little less specific but it is more sensitive: less than 1 ng/ml of IgG can be detected. The number of IgG molecules/platelet detected investigating normal and pathological sera is higher than that reported by others: such results are probably related to the nonhomogeneity of immunoglobulins implicated in the assay.