Supravital preparations of blood and haemopoietic tissues have been studied by interference contrast, phase contrast and conventional light microscopy to help identify the different categories of the smaller blood cells of lampreys. These methods were also used in conjunction with an examination of sectioned material and fixed blood films, to investigate macrophage activity by the blood cells and the liver Kupffer cells. The diameter of the smallest living blood cells was 4–5 |mUm. When these produced extensive clotting fibres, particularly in the presence of foreign substances, they were obviously thrombocytes, whereas others, some of which contained prominent nucleoli, were considered to represent small lymphocytes and stem cells. Slightly larger cells (5–6 |mUm), in which the nuclearcytoplasmic ratio was slightly lower and granules were being formed, represented the first stage in the formation of neutrophilic and eosinophilic granulocytes. The early members of the erythrocyte lineage could be distinguished from these early progranulocytes by their greater thickness under interference contrast microscopy and by their much more rapid degeneration and lack of amoeboid movement in blood films. All blood cells, apart from erythrocytes and differentiating cells of the erythrocyte series, showed amoeboid movement and many were capable of phagocytosing small particles. There was no evidence that, except for macrophages derived from lymphocyte-like cells, any free blood cells were capable of engulfing whole erythrocytes. During metamorphosis, phagocytic activity was very conspicuous in both blood cells and Kupffer cells, the latter of which exhibited pronouncedn erythrophagocytosis.