Values of inhibition constants, K_i, and concentrations required for 50% inhibition, IC₅₀, for a collection of structurally diverse competitive inhibitors of calf spleen purine nucleoside phosphorylase have been determined employing inosine as substrate. These values have been employed to create predictive quantitative structure-activity relationships (QSAR) which link structure to values of K_i and IC₅₀. These QSAR models have substantial power to predict values and the associated uncertainties for K_i and IC₅₀ for unknown, structurally diverse inhibitors of purine nucleoside phosphorylase.