Detection of expression of the La^a epitope on recombinant GPIIb-IIIa complexes. Expression on Cos-7 cells of GPIIb-IIIa complexes containing wild-type (A) or mutated (B) GPIIIa was determined using E.V.'s serum as a positive control (▧), nonimmune serum as a negative control (□), and C.M.'s serum (▧). The optical density (OD) was read 16 hours after addition of the chromogenic substrate. Values are the mean of 6 points obtained in two different experiments. The same number of cells were analyzed for each serum tested.