The Fifth International Society of Blood Transfusion (ISBT) Platelet Serology Workshop took place in 1990. A total of 21 laboratories participated in the wet workshop. Results were discussed at a meeting of the International Committee on Standardization in Haematology (ICSH) Expert Panel on Platelet Serology at the Joint Congress of the ISBT and the American Association of Blood Banks in Los Angeles in November 1990. Each of the participants analyzed 37 serum samples for platelet-reactive alloantibodies and isoantibodies. In addition, two drug-dependent (quinine) platelet-specific antibodies had to be identified among a subset of seven sera. For the first time in a platelet serology workshop, five platelet suspensions had to be typed for the alloantigens of the HPA-1, -3, -4, and -5 systems.