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 a- and [S-subunits. The distribution of equine inhibin activity from the initial sizing column (S-200, 25 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, -subunit bands were detected at 40 000 M,, 56 000 Mr, 80 000 Mr, and 90 000 M,; Pa reactive bands were identified at 56 000 Mr (strong) and 90 000 M, (faint). Western blot analysis of the lower-molecular-weight inhibins on one-dimensional (ID) SDS-PAGE gels revealed one inhibin band at 32 000 Mr. In reduced ID-PAGE gels, a doublet a-subunit band was found at 18000 M,, and one pa band was found at 14 000 M,. The 18 000 M, equine a-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 a-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,, 56 000 M,, and 32 000 Mr. Alpha-subunit-only bands at 40 000 Mr and 80 000 M, were detected. The lower-molecular-weight form of equine inhibin is similar to porcine inhibin in size and pattern on 2D-PAGE.