Reversible protein phosphorylation plays a central role in numerous biochemical pathways and functions to alter protein activity and/or conformation (3,7). Methods for detecting protein phosphorylation have predominately relied upon the use of radioactive 32P for either the in vitro or in vivo phosphorylation reaction (2,6). To identify the specific amino acid residues that become phosphorylated on a specific protein, phosphoamino acid analysis is then required (2,3,6,7). The use of radioactivity and the multi-step analysis process which follows makes the entire process both hazardous and tedious. Over the past decade, antibodies specific for phosphotyrosine were developed, many of which can detect a single phosphorylated tyrosine residue (1,4,5). Although antibodies to phosphotyrosine have been relatively straight forward to produce, the extensive structural similarity between phosphoserine and phosphothreonine has contributed to the difficulty in raising highly specific antibodies to these phosphoamino acids (6).