ABSTRACT. Immune function in cows is closely associated with their physical and hormonal conditions. In order to clarify the relationship between the body condition score (BCS) of lactating dairy cows and the immune response to progesterone (P 4 ) in vitro, we examined whether lower BCS in dairy cows affects the responsiveness of peripheral blood mononuclear cells (PBMCs) to P 4 added in to culture medium. Forty-two non-pregnant healthy Holstein dairy cows were examined at 61 to 120 days after calving. The cows were divided into the following two groups; Low BCS group (N=20), which had a BCS of less than 2.25, and a Control group (N=22), which had a BCS over 2.75. PBMCs were stimulated with P 4 (1 g/mL) and/or phytohemagglutinin (PHA), and the levels of cytokine mRNA were analyzed. In the Low BCS group, a significantly lower IFN- level was stimulated by PHA only compared with the Control group. The combination of P 4 and PHA significant decreased the IFN-/IL-4 ratio in the Control group, but this reaction was not found in the Low BCS group. Our data indicated that expression of IFN- mRNA was basically lower in the low BCS dairy cows and that addition of P 4 did not suppress the cellular immune function in these cows. In this study, we observed that P 4 reduced the cellular immune response in the adequate BCS cows, whereas immunosuppression by P 4 was not found in the PBMCs of the low BCS cows, which already had a lower level of immune function. KEY WORDS: body condition score, dairy cow, immune function, progesterone.J. Vet. Med. Sci. 71 (5): [549][550][551][552][553] 2009 Mastitis and endometritis are frequently observed in dairy cows, and these diseases affect productivity. Yield and quality of milk decrease in mastitic cows, and reproductive performance declines in cows with endometritis. Deluyker et al. [4] reported that occurrence of clinical mastitis is associated with 5% milk yield loss and that cows with a mean SCC greater than 245,000 cells/ml over 119 d show a 6.2% yield loss compared with cows with an SCC of 90,000 cells/ ml or less. Failure of immune function to prevent bacterial invasion in the mammary gland or uterus is closely related to the occurrence of these infectious diseases [11]. Low body condition score (BCS) is closely associated with morbidity in the dairy cow [6,20], and a higher somatic cell count (SCC) or clinical mastitis is frequently observed in low BCS cows [21].It is well known that cows with a BCS of less than 2.25 have reduced breeding performance [5,15]. Progesterone (P 4 ) is important for fertility as demonstrated by the positive correlation between the serum P 4 level at the time of artificial insemination and the subsequent conception rate. Therefore, P 4 supplementation is used frequently to improve the serum P 4 level in order to enhance the reproductive performance of dairy cows with low serum P 4 levels, such as lower BCS cows [18]. On the other hand, P 4 is well known as one of the hormones that suppresses immune function. Previous studies in the mouse have re...