The immunoreactivity of five sera containing either iso-, auto- or allo-antiplatelet antibodies to frozen-thawed human lymphocyte antigen (HLA)-typed pooled platelets was assessed to determine the practicality of using such a platelet preparation in an immunofluorescent platelet antibody system. The platelet reagent was analyzed over an 8-week period after initial freezing and storage. Mixtures of platelets with each serum or dilutions of it were initially incubated for 1 hour, then extensively washed and incubated with fluorescein-conjugated goat F(ab')2 antihuman IgG, A, M. After repeat washing, each mixture was then analyzed using a flow cytometer to determine the relative amount of fluorescent antibody bound. Not only was each of the five sera consistently positive for the presence of an anti-platelet antibody, but the extent of immunoreactivity for each remained relatively stable over the 8-week period of analysis. The results suggest that frozen pooled HLA-typed platelets should be an invaluable reagent for screening serum samples for the presence of antiplatelet antibodies in patients who are thrombocytopenic or who are refractory to platelet transfusion therapy.