Mammary glandular tissues and mammary secretions were obtained from sheep at 2-60 d after weaning to study the leucocyte phenotypes associated with mammary involution. From 2-4 d after weaning, neutrophils were the predominant leucocytes in the alveolar and ductal lumina. Lymphocytes were present in the alveolar and ductal epithelium, interalveolar and periductal areas. Most of the lymphocytes in the alveolar and ductal epithelium (IEL) were CD8+, some were CD45R+ and few were CD4+. In the periductal clusters and in the interalveolar areas most of the lymphocytes were CD4+. There was a significant increase (P < 0.05) in the percentages of CD45R+ granulated IEL from 2 to 7 d after weaning, and this paralleled the increase in the percentages of apoptotic cells in the glandular epithelium. By 7-60 d after weaning, most cells within the alveolar and ductal lumina were macrophages followed by predominantly CD8+ lymphocytes. CD8+ lymphocytes were still predominant in the alveolar and ductal epithelium while CD4+ cells were predominant in the interalveolar areas. Very few gammadelta+ T cells were observed at all the stages examined. The cells in the mammary secretions correlated with those observed in the alveolar and ductal lumina. At the early stages of involution, the neutrophils and macrophages were heavily laden with lipid droplets, casein and cellular debris. The most interesting feature was the presence of cells either with extensive cytoplasmic processes (LCA+MHC class II+) or cytoplasmic veils (LCA+MHC class II+CD1+), probably dendritic cells. It is concluded that the cellular constituents of the mammary gland at the latter part of involution may afford the mammary gland more resistance to infection than the lactating gland and the gland at early stages of involution. The CD45R+IEL may trigger apoptotic cell death in the mammary glandular epithelium during mammary involution.