Data from the 1977 College of American Pathologists (CAP) Chemistry Survey Program have been analyzed in order to evaluate currently used laboratory methods for determination of serum and urinary phosphate concentrations. The methods may be divided into those based on (1) molybdate and a reductant, (2) molybdate without a reductant (UV), (3) dye-binding, and (4) enzymatic (UV) assays. The majority of CAP Survey participants use a molybdate and reductant technic. Procedures using other reductants such as ferrous sulfate, ascorbate, and p-methylaminophenolsulfate manifest a slight positive bias. Compared to an overall mean concentration index, the SMAC-UV method manifests a considerable negative bias.