We studied the biological activity of a recently characterized 31-amino acid inhibin-like peptide (ILP) using a synthetic preparation. While the material yielded a single component on chromatography, amino acid sequence analysis suggested that only 30% of the molecules possess the complete structure. Bioactivity was tested in vitro using whole pituitaries from 25 day-old male Sprague-Dawley rats. Pituitaries were incubated with 500 ng/ml of the ILP preparation or vehicle alone for 60 min, followed by a 3 h exposure to 2 ng/ml of luteinizing hormone releasing hormone (LHRH) or its diluent. In the ILP-incubated pituitary media, no significant suppression of basal FSH and LH release or stimulated FSH release was observed, while a significant increase in stimulated LH release was seen (p less than 0.05). An in vivo bioassay study was performed on 38 day-old male Sprague-Dawley rats. Test animals were injected with 0.1, 1, 10 or 20 mcg of ILP immediately after castration, and 10 and 24 h later. Control animals received bovine serum albumin or vehicle alone. There were no statistically significant differences in serum LH or FSH concentrations taken at 30 h after castration between the ILP-treated rats and controls. Possible reasons for our inability to demonstrate inhibin bioactivity (selective FSH suppression) with this ILP preparation include: Only about 30% of the peptides had the complete amino acid sequence. Other peptides with deletions might have acted as antagonists, thus obscuring the inhibin bioactivity. The 31-amino acid ILP may not be the authentic inhibin molecule. We suggest that further studies are needed to determine the true identity of inhibin.