We analyzed the results of 71 leukapheresis procedures performed in 21 patients to identify the best predictive factors affecting the yield of peripheral blood progenitors after high-dose chemotherapy followed by G-CSF administration. An average of 1 +/- 1 x 10(8) MNC/kg, 5 +/- 6 x 10(4) CFU-GM/kg and 4 +/- 6 x 10(6) CD34+ cells/kg was collected for each leukapheresis. When we defined > or = 5 x 10(4)/kg as the minimum number of CFU-GM per procedure for a 'satisfactory' collection, multiparameter analysis of clinical features and laboratory findings showed that the only factors that predicted the numbers of CFU-GM collected were prior treatment with the MOPP regimen and the number of mononuclear cells identified in the basophil channel of the H*1 = Technicon. A logistic regression analysis performed to generate a mathematical model revealed four predictive factors: the number of previous cycles of chemotherapy, previous MOPP chemotherapy, the interval from latest chemotherapy and the number of mononuclear cells/microliter. This model was valuable in defining the optimal time for the first leukapheresis procedure. In contrast, the number of circulating CD34+ cells did not correlate with CFU-GM numbers collected whereas the numbers of mononuclear cells did provide a simple and reliable index. Thus the principal factor affecting the efficiency of peripheral blood stem cell collection was prior therapy with MOPP.