The immunoreactivity of a PLA1 antibody-containing serum to frozen/thawed platelets prepared by four different procedures was measured to assess the practicality of preparing a platelet reagent that could be easily stored and readily used in an immunofluorescent platelet antibody detection system. These frozen aliquots were analyzed at intervals up to 23 weeks after initial freezing and storage. For analysis, the frozen platelets were thawed, washed, resuspended to 2.5 x 10(5) platelets/mL and incubated with a dilution of fresh autologous serum that lacked platelet antibody or with a similar dilution of frozen human serum containing IgG anti-PLA1 antibody. At each interval, a freshly drawn platelet sample from the same donor was incubated in the same manner and used for comparison. All platelet mixtures were then washed and incubated with fluorescein conjugated goat F (ab')2 antihuman IgG. After repeat washing, each mixture was then analyzed with a flow cytometer for the extent of fluorescent antibody bound to each platelet mixture. Anti-PLA1 antibody reactivity with either the fresh or the frozen/thawed platelets remained stable over the period of analysis, with no significant difference in immunofluorescence with either fresh or frozen platelets as target cells. Platelet recovery following the thawing step ranged from 12% to 36% and was independent of the storage time. These studies suggest that frozen platelets can be readily used as reagents for platelet antibody assays.