The serum from an 85-year-old man with a clinical diagnosis of post-transfusion purpura (PTP) was investigated for the presence of platelet-specific antibodies. Clinically, the case was typical of PTP but, unusually, the serum was found to contain multiple platelet-specific alloantibodies. Anti-HPA-1a, anti-HPA-2b and anti-HPA-3a antibodies were detected together with multispecific anti-HLA (class I) antibodies. Additional (but weaker) antibody reactivity was also observed with platelet glycoproteins (Gp) IIb/IIIa, GpIb and GpIa/IIa which lacked the antigens recognized by the alloantibodies, suggesting the presence of auto- or cross-reacting antibodies. The patient's genotype was HPA-1b/1b, HPA-2a/2a, HPA-3b/3b, HPA-5a/5b and was consistent with the platelet alloantibodies detected. The patient made a complete recovery following treatment with intravenous gamma-globulin.