Western blot hybridized with monoclonal antibody PY20 to phosphotyrosine. The SDS gel was loaded with 1 μg protein per lane. 1) lysozyme-PYMRR-OP, 2) BSA-LYGLPR-OP, 3) human albumin-QYDVRK-OP, 4) human albumin-YGGFL-OP, 5) control human albumin, 6) tubulin-OP, 7) control tubulin, 8) and 9) O-phospho-L-tyrosine-crosslinked to human albumin. Monoclonal antibody PY20 specifically recognized phosphotyrosine-labeled protein, but did not recognize diethoxyphosphorylated tyrosine labeled proteins.