The zeta sedimentation ratio (ZSR) is essentially a rapid micromethod with integral hematocrit correction for the determination of the erythrocyte sedimentation rate (ESR). A quality control method for the ZSR procedure is described which identifies the problems most likely to occur during the analysis. The method requires the analysis of an EDTA-anticoagulated, undiluted, whole blood sample in a standard Westergren tube. From the table supplied, the corresponding ZSR is determined and the difference, if any, is plotted. Typical patterns diagnostic of various analytical problems are illustrated.