Bromodeoxyuridine, an analogue of thymidine, can be detected by means of monoclonal antibodies and utilized as a marker of the S-phase of the cell cycle. In this paper a method for the detection of the labeling index of normal and neoplastic colony-forming units (CFU) growing in plasma clot semisolid medium is described and preliminary results on the cell cycle of 7th and 14th CFU granulocyte-macrophage are discussed.