Abstract. This study was carried out to evaluate the blood profile and tissue expression of Anti-Müllerian hormone (AMH) as a biomarker for granulosa-theca cell tumors (GTCTs) in cattle. Five cases with unilateral ovarian GTCTs (GTCT group) were investigated in comparison to other groups of Japanese Black cows, which had either cystic ovarian disease (COD group, n=5), a functional corpus luteum on Days 9 to 11 of the estrous cycle (Day 0=estrus; CL group, n=13) or received superovulation treatment (SOT group, n=13). We used transrectal ultrasonography and measured plasma AMH, estradiol-17β (E 2 ), progesterone (P 4 ) and testosterone (T) levels. Moreover, GTCT tissues were collected and examined by immunohistochemical staining (IHC) for AMH. In the GTCT group, ultrasound images of GTCTs were variable and not definitive. However, the AMH level in the GTCT group (n=3, 58.1 ± 66.3 ng/ml) was significantly higher than in the COD, CL and SOT groups (0.1 ± 0.1 ng/ml for GTCT vs. COD, P<0.05; 0.2 ± 0.1 and 0.3 ± 0.2 ng/ml, respectively for GTCT vs. CL and SOT, P<0.01). The other hormonal levels in the GTCT group had no significant differences compared with the COD or SOT group. Neoplastic granulosa cells labeled with AMH antibody clearly demonstrated a variety of tissue patterns in all cases by IHC. To the best of our knowledge, this is the first study to investigate the blood profile and IHC of AMH in bovine GTCTs. Our findings indicate that AMH may be a novel biomarker to diagnose GTCTs in cattle. Key words: Anti-Müllerian hormone (AMH), Cattle, Granulosa-theca cell tumors (GTCTs), Immunohistochemical staining (J. Reprod. Dev. 58: [98][99][100][101][102][103][104] 2012) G ranulosa-theca cell tumors (GTCTs) are the most common ovarian tumors in cattle [1], and the incidence may be less than 0.5% [2]. GTCTs can affect various breeds [3] and occur at various ages [4] in cattle. GTCTs produce a variety of steroid hormones, causing a subsequent elevation of plasma estradiol-17β (E 2 ), progesterone (P 4 ) and/or testosterone (T) levels [4]. Therefore, a variety of clinical signs, such as nymphomania [5], virilism [6] and mammary gland development [7], could be presented. In contrast, some in GTCTs may not result in abnormal reproductive behavior at all [3]. Preliminary diagnosis of GTCTs can be achieved by transrectal palpation and ultrasonography [1,4]. GTCTs should be suspected if a chronic cystic ovarian disease (COD) does not respond to standard treatment regimens or if the diameter of the ovary is more than 100 mm [1]. Thus, GTCTs should be distinguished from other conditions, such as COD, oophoritis, ovarian abscesses and parovarian cysts, by these clinical signs [8]. However, the distinctions might be difficult depending on the clinical signs and other diagnostic methods, so a definitive diagnosis can only be made based on histopathological examination of the affected ovary [3]. In the treatment of unilateral bovine GTCTs, unilateral ovariectomy is only indicated in cows that do not exhibit alterations in secon...