Protein phosphatases play significant roles in signal transduction pathways pertaining to cell proliferation, gene expression, and neurotransmission. Serine/threonine phosphatases PP1 and PP2A, which are closely related in primary structure (approximately 50%), are inhibited by a structurally diverse group of natural toxins. As part of our study toward understanding the mechanism of inhibition displayed by these toxins, we have developed research in two directions: (1) The standardization of an assay to be used in acquisition of the structure--activity relationship of inhibition data is reported. This nonradioactive assay affords detection levels of molecular phosphate released from a phosphorylated hexapeptide in subnanomolar quantities. The comparison of our IC50 values of these inhibitors against corresponding literature data provided validation for our method. (2) Computational analysis provided a global model for binding of these inhibitors to PP1. The natural toxins were shown to possess remarkably similar three-dimensional motifs upon superimposition and van der Waals minimization within the PP1 active site.