AIMS: To determine the frequency of immature haemopoietic cells in the peripheral blood of healthy persons. METHODS: Cytocentrifuge preparations were made using mononuclear leucocytes separated by a Ficoll-Hypaque density gradient. The slides were stained by May-Grünwald-Giemsa. The combination with immunoperoxidase technique allowed immunotyping of uncommon blood cells. RESULTS: Blast cells expressing the progenitor cell marker CD34 represented 0.11 (0.06) per cent (mean (SD)) of the total mononuclear leucocyte count; these were the haemopoietic progenitor cells in the peripheral blood. Dark blue cells expressing CD38, CD45, HLA-DR, CD4, CD11a, CD29, CD49d, CD50, and CD54 represented 0.30 (0.21) per cent of the mononuclear leucocytes; most of these cells did not express T, B, NK, myelomonocytic, progenitor cell, proliferation, activation, blood dendritic cell, or follicular dendritic cell markers. These were dendritic cell precursors in the peripheral blood. Very small numbers of cells expressing CD83 were found. Blast-like cells expressing CD45, HLA-DR, CD11a, and CD50 represented 0.15 (0.10) per cent of the mononuclear leucocytes; morphology and immunotyping supported the conclusion that these cells were poorly differentiated monocytes. CONCLUSIONS: Morphological investigation of mononuclear leucocytes in peripheral blood of healthy persons can be used to detect small numbers of blasts, dark blue cells, and blast-like cells. The immunoperoxidase technique can then be used for immunotyping of these cells. This simple method may be helpful in diagnosing haematological disorders.