Reduced response of PV BFU-E to vanadate. Day-1 BFU-E were prepared as previously described9 and were plated at 100 to 200 cells/mL in plasma clots with serum medium, 2 U/mL rhEP, 50 U/mL rhIL-3, and increasing concentrations of vanadate. After 14 days of culture, the clots were fixed and stained with benzidine-hematoxylin.29 Each value is the mean ± SD of four (normal) or three (PV) experiments expressed as a percentage of the controls without vanadate, which were set at 100. The purity of the normal BFU-E was 48% ± 6%, 31% ± 4%, 49% ± 5%, and 59% ± 3%, whereas the purity of the PV BFUE was 43% ± 4%, 38% ± 4%, and 50% ± 8%, respectively.