The development and validation of a method for the determination of concentrations of thiocyanate in human plasma are described here. A modified colorimetric method of Bowler was used with the following alteration in Monica Manual, Part III. In order to obtain the same sensitivity in low amounts of clinical samples, quartz SUPRASIL(R) micro cuvettes have been used. The quantitation range was between 25-500 microM. Accuracy and precision of the quality control samples, linearity of the calibration curve, dilution, spike recovery and stability under various conditions were evaluated in the validation of the method and all demonstrated acceptable results. All validation results met good laboratory practice acceptance and FDA requirements to be acceptable for application in clinical trials. The validated method has been used for a Phase I clinical study in cancer patients orally administered with either 60 mg or 80 mg of GDC-0425 containing a cyanide (CN-) group. The thiocyanate levels from patients before and after drug administration showed no clinically significant differences.