A validation study of the DPD activity measurement was carried out using peripheral mononuclear cells obtained from two healthy male adults. As a result, the DPD activity was 450.2 pmol/min/mg protein (mean value of 5 measurements), the intra-day reproducibility (C.V. intra.) was 12.5%, and the inter-day reproducibility (C.V. inter.) was 24.5%, indicating rather large deviations. The mean of the recovery rate in this study was, however, 100.4%, which is almost 100%, and the deviation (C.V. = 4.2%) was small. The HPLC apparatus and the analysis devices worked with good precision and reproducibility. It could be predicted that the DPD activity values might vary during the collecting process of monocytes. A large variation might have resulted from the large variance in the cellular composition of the mononuclear cell fraction.