Human erythrocytes were incubated without or with 1 mmol/L pervanadate, 2 mmol/L diamide, and 1 mmol/L NEM in the absence or presence of 5 μM PP2. Isolated erythrocyte membranes were treated with extraction buffer, and extracted proteins were immunoprecipitated with anti-Syk or anti-Lyn antibodies. Tyrosine kinase activities of immunocomplexes were tested in vitro, as described in “Materials and methods.” Reported values represent the means for 4 separate experiments. SEMs were always less than 20%. IP indicates immunoprecipitate; NEM, N-ethylmaleimide.