cDNAs encoding equine inhibin/activin beta A subunit were isolated from an equine follicle cDNA library and characterized. Using primers based on the rat inhibin/activin beta A subunit cDNA sequence, a RT-PCR was performed to generate the probe for screening. Four positive clones were isolated. Analysis of the nucleotide sequence of these clones revealed that two pairs of identical clones were present, Eq-beta A-1 (0.9 kb) and Eq-beta A-2 (1.5 kb). Eq-beta A-2 clone contained a complete open reading frame encoding 426 amino acids. The deduced amino acid sequence of equine inhibin/activin beta A subunit showed high similarity (> 90%) to those of five other mammalian species. Northern blot analysis revealed that placenta from mare on day 180 of pregnancy contained a 1.5 kb inhibin/activin beta A subunit mRNA.