- A variety of assays are available for measuring platelet-associated IgG (PAIgG) but the complexity of these assays increases the potential for technical errors. These errors are difficult to detect and, if possible, known positive and negative control platelets should be included with each run. However, patient platelets with elevated levels of IgG are seldom available. This report describes a method for producing positive control platelets that can be labeled with differing amounts of IgG. Normal serum IgG (Cohn fraction II) was incubated with washed 2 percent formalin-fixed platelets for 60 minutes at 37 degrees C. The amount of IgG on the platelets was proportional to the concentration of soluble IgG and the number of incubations. Normal platelet IgG levels were 1.2 +/- 0.1 fg per platelet (mean +/- SEM, n = 34) and positive control platelets had 4.6 +/- 0.2 (n = 12) or 8.4 +/- 0.4 (n = 7). There was no change in the level of PAIgG when stored at 4 degrees C for 1 week, although there was a 28 percent loss in recoverable platelets. When mixed 1:1 with 60 percent glycerol and stored at -70 degrees C, the level of PAIgG was stable for 3 months, with less than 12 percent platelet loss on recovery (n = 12). These positive control platelets have proved useful for monitoring assay performance.