The leucocytes in sheep colostrum, milk and involution secretion were studied using the electron microscope. The predominant cell type in colostrum was the polymorphonuclear leucocyte (PMNL) (41–84%), followed by the macrophage (8–49%), and the lymphocyte (6–11 %). Plasma cells were present in low numbers (1–2%) while no secretory epithelial cells were observed. In mid-lactation the cell components changed so that the macrophage was the predominant cell (83–86%) followed by the lymphocyte (10–17%). In early involution secretion, PMNL reappeared but declined in secretion obtained 21 d after weaning, when the macrophage was again the predominant cell. Associated with the whole cells were membranous extracellular materials and ‘sunburst’ fragments of the secretory epithelial cells. These were phagocytosed by both PMNL and macrophages, which also engulfed fat droplets from the milk. The lymphocytes were examined for plasma membrane markers and the T-cell percentage (E+ rosettes) fluctuated within wide limits, (0–80 %). However, there was no discernible trend associated with stage of lactation. The B-cell markers (C1 and Fcγ) also fluctuated widely and it was evident that contaminating monocytes reduced the accuracy of the count. However, it appeared that more B-cells were present in milk than in blood.