Three new patients with post-transfusion purpura (PTP) are described. As the manifestations in two differ significantly from those of previously reported cases, they serve to expand the definition of this syndrome. Although all 14 previously reported cases have occurred in Pl- A1-negative females, one of our patients was a Pl-A-negative male. Moreover, a female whose postrecovery platelets possessed the Pl-A1 antigen is described. Antiplatelet antibody activity was detected in all three patients by the 51Cr release test; in contrast, only one reacted in the complement (C) fixation assay. Serum obtained during the acute episode from the PlA1-positive patient reacted against platelets from four of 11 normals by C fixation and against platelets from 48 of 53 normals by 51Cr release, including five of nine Pl-A1-negative platelet samples. This case represents the first instance of PTP in which the platelet isoantibody was not specifically directed against the Pl-A1 antigen. These observations suggest that PTP may be a more heterogeneous disorder than previously realized.