Correlations between different peripheral blood parameters and harvesting yields of stem cells were analysed in 230 leukaphereses performed on 61 patients. According to multivariate analysis, the number of circulating CD34 positive cells was found to provide the best preleukapheresis parameter for predicting the quality of peripheral blood progenitor cell harvest. A high correlation coefficient and small 95% confidence and prediction intervals would indicate the assay of CD34 positive cells in peripheral blood to be a unique tool essential for monitoring the collection of circulating haematopoietic progenitors.