Colony‐forming cells (CFC) and long‐term culture‐initiating cells (LTC‐IC) include a spectrum of progenitor types whose potential contributions to the haemopoietic recovery seen in patients transplanted with mobilized peripheral blood progenitor cells (PBPC) remains unclear. We evaluated both the number and cycling status of the circulating LTC‐IC and CFC harvested from 12 patients treated with chemotherapy and G‐CSF using a modified 6‐week LTC‐IC assay. The frequency of the LTC‐IC and CFC in the mobilized PB samples were increased 45‐ and 750‐fold, respectively. Interestingly, comparison of these values for PB samples, taken just prior to the start of the leukapheresis, with the progenitor content of the 3 h harvest, showed that, on average, the leukapheresis product contained 19 times more LTC‐IC (P < 0.01) than had been detectable in the entire blood volume of the patients at the start of the collection, whereas the number of CFC collected was approximately the same as the number in the initial circulating pool of PBPC. Cycling studies showed many of the LTC‐IC in the apheresis collections to be proliferating although not more so than in the steady‐state marrow LTC‐IC compartment (i.e. per cent kill of mobilized LTC‐IC after 16 h in ³H‐Tdr = 70 ± 8%, n = 9). On the other hand, the majority of the CFC in the apheresis collections were initially quiescent (per cent kill after 16 h in ³H‐Tdr = 37 ± 6%, n = 12). These findings demonstrate the rapidity with which a primitive subset of LTC‐IC may enter the circulation during the early phase of rebound haemopoiesis induced by chemotherapy plus G‐CSF and provide evidence of differences in the mechanisms regulating LTC‐IC and CFC mobilization.