Immune cells and their cytokine products have powerful local effects within body tissues. There has been great interest in the potential role of these cells, not only during destruction of the corpus luteum but also during its functional lifespan. In this study, lymphocytes, macrophages and major histocompatibility complex class II molecules were quantified using immunohistochemistry and the reverse transcription-polymerase chain reaction was used to detect mRNA for tumour necrosis factor alpha and interferon gamma within corpora lutea from three groups of cows: (1) corpora lutea collected at an abattoir and assessed visually into four stages (stage I (days 1-5), stage II (days 6-12), stage III (days 13-18) and stage IV (days 19-21) of the oestrous cycle); (2) corpora lutea collected around natural luteolysis (days 14-20); and (3) corpora lutea collected 6, 12 and 24 h after prostaglandin F 2 alpha-induced luteolysis. The numbers of T lymphocytes (CD5+ and CD8+) were significantly higher (P < 0.05) at stage IV and from day 16 onwards, before functional luteolysis. There were significantly higher numbers (P < 0.01) of macrophages at stages I, III and IV compared with stage II in visually staged tissue. Major histocompatibility complex class II molecules were increased (P < 0.05) at stages I and IV compared to stage II and at all times after induced luteolysis. Using reverse transcription-polymerase chain reaction, mRNA encoding tumour necrosis factor alpha and interferon gamma was detected in all luteal tissue collected around natural luteolysis and after induced luteolysis. These findings, particularly the increase in T lymphocytes before functional luteolysis, provide further evidence of a significant role for the immune system in affecting reproductive function in cows.