Inhibin has been characterized from a number of mammals; however, it has not been extensively studied in horses. Western blot analysis was used to examine the size heterogeneity of equine inhibin alpha- and beta-subunits. The distribution of equine inhibin activity from the initial sizing column (S-200, 25 x 94 cm) indicated that the majority of equine inhibin activity was present as larger-molecular-size forms. When the large forms were analyzed by Western blot in nonreducing conditions, alpha-subunit bands were detected at 40,000 M(r), 56,000 M(r), 80,000 M(r), and 90,000 M(r); beta a reactive bands were identified at 56,000 M(r) (strong) and 90,000 M(r) (faint). Western blot analysis of the lower-molecular-weight inhibins on one-dimensional (1D) SDS-PAGE gels revealed one inhibin band at 32,000 M(r). In reduced 1D-PAGE gels, a doublet alpha-subunit band was found at 18,000 M(r), and one beta a band was found at 14,000 M(r). The 18,000 M(r) equine alpha-subunit was present in three distinct spots in the isoelectric focusing (IEF) dimension of two-dimensional (2D)-PAGE, and closely overlapped those of porcine inhibin alpha-subunit. In conclusion, inhibin is present in good yield in equine follicular fluid. A higher proportion of the total activity is present in higher-molecular-weight forms than with porcine inhibin. Inhibin was detected at 90,000 M(r), 56,000 M(r), and 32,000 M(r). Alpha-subunit-only bands at 40,000 M(r) and 80,000 M(r) were detected. The lower-molecular-weight form of equine inhibin is similar to porcine inhibin in size and pattern on 2D-PAGE.