Lymphocytotoxicity test (LCT) and platelet suspension immunofluorescence test (PSIFT) were used together to screen platelet-associated antibodies in patients who received long-term platelet transfusion. Twenty-four of 53 patients (45.3%) were immunized subsequently. Since the concordance of LCT and PSIFT was 100%, most of the platelet associated antibodies were of HLA specificity, and platelet specific antibody alone (in absence of HLA) was not detected. The identified antibodies were anti-A2, A11, A24, B5, B46, B57, B60, and B62. The majority of them were against the high frequency HLA antigens in the Chinese population. The development of antibody could not be correlated with the number of platelet-donors exposed, the time interval after the initiation of platelet transfusion, or the percentage of reactive lymphocytotoxic panels. HLA antibody was the major factor in causing platelet alloimmunization in the Chinese patients. However, some other unknown factors should be looked for. In addition, ABO incompatibility did not affect the posttransfusional increment while the platelet was compatible with LCT crossmatching.