Using a limiting dilution assay the frequency of long-term culture-initiating cells (LTC-IC) in the apheresis products following mobilization by granulocyte-colony stimulating factor (G-CSP) with or without chemotherapy from 14 normal donors (ND) for allogeneic bone marrow transplantation, 16 patients with multiple myeloma (MM) and 15 patients with acute myeloid leukaemia (AML), where the aphereses were intended for autologous transplantation, were compared. The estimated median incidences of LTC-IC in the first apheresis products from ND, MM and AML were 1/3289, 1/1775 and 1/13075 mononuclear cells (MNC) respectively. The patients with AML had a significantly lower incidence compared with the other two groups (P < 0.0001). There was a positive correlation between the incidence of LTC-IC and the number of CD34+ cells, the number of GM-CFC, and the number of BFU-E. The positive association with GM-CFC or BFU-E was weaker. In these experiments the percentage of CD34+ cells was the best predictor for the frequency of LTC-IC in the peripheral blood progenitor cells (PBPC). In eight cases of MM the LTC-IC assay was performed for both the first and second harvest. All cases had a lower LTC-IC frequency in the second harvest compared with the first, an average of 23% (13-42%, 95% confidence interval) and this reduction was statistically significant (P<0 001); CD34+ cells were also lower (P< 0.001).