SummaryA chemiluminescence technique (CLT) has been developed which measures the interaction between human monocytes and antibody‐coated (opsonized) platelets. This technique has an objective end‐point, is simple to perform and is of comparable sensitivity to the platelet suspension immunofluorescence test (PSIFT) when used to detect anti‐platelet alloantibodies. In contrast, only 4/20 sera from patients with clinically diagnosed autoimmune thrombocytopenia were opsonic in the CLT, while 8/20 of these same sera bound IgG to platelets in the PSIFT. Only one serum gave positive results in both