In peripheral blood progenitor cell (PBPC) collections from patients with solid tumour or haematological malignancy, monocytes were separated into two subpopulations. The majority of monocytes expressed CD14 at a high density without CD16 antigen (the CD14+CD16- monocytes). The remaining monocytes co-expressed CD14 and CD16 (the CD14+CD16+ monocytes). These CD14+CD16+ monocytes amounted to 20.6 +/- 15.8%, while those in peripheral blood (PB) obtained from healthy volunteers were 7.3 +/- 3.1% (P < 0.05). When subdividing the CD14+CD16+ monocytes into CD14brightCD16dim and CD14dimCD16bright cells, both populations were found to be increased in PBPC collections. Since typical CD14+CD16+ monocytes are the CD14dimCD16bright population, we compared the additional surface antigens on CD14dimCD16bright monocytes with those of CD14+CD16- monocytes. In PBPC collections, the CD14dimCD16bright monocytes exhibited lower levels of CD11b, CD15, CD33 and CD38 expression and higher levels of CD4, CD11a, CD11c and MHC class II, and also revealed a higher percentage of CD4+ cells and a lower percentage of CD15+ cells and CD38+ cells, compared with the CD14+CD16- monocytes. When compared with the CD14dimCD16bright monocytes in PB, those in PBPC collections exhibited higher expression of CD4 and lower expression of CD11b, and also showed higher percentages of CD4+ cells and CD38+ cells and a lower percentage of CD11b+ cells. These results suggest that PBPC collections may be rich in the CD14+CD16+ monocytes in which the proportion of the immature population is increased. It is likely that these monocytes participate in the haematological and immune recovery after PBPC transplantation.