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SummaryExtracts of porcine pituitaries can be separated into two fractious, each containing immunoreactive growth hormone, (GH) by gel filtration on Sephadex G-200. The void volume peak contained 0·2–0·5% (large GH) and the remainder fluted at Kd= 0·6 (small GH). 50–60% of the immunoreactive large CH was absorbed on to a large excess of anti-GH coupled to Sepharose 2B during an 18 hour incubation at 4°. Over 80% of the absorbed immunoreactivity eluted with neutral 6 M NaSCN at 4°. The pituitary extracts also contained RNA which was absorbed and eluted along with the large GH. Upon rechro-matography on Sephadex G-200 the eluate contained small GH, suggesting that large GH was dissociated to small GH. The RNA ran in the void volume. RNA was also non-specifically absorbed when Sepharose coupled with bovine serum gamma globulin was substituted for anti-GH Sepharose. However, interference by RNA could be avoided if large GH was converted to small GH using 6 M NaSCN and rechromatographed before immunoabsorption. Contamination by prolactin was found.Following a 2 hour incubation of porcine pituitary slices with (3H) leucine, the specific activity of large GH was 10–100 times that of small GH. It is concluded, firstly, that pituitary large GH is not small GH covalently bound to RNA, and, secondly, that pituitary large GH represents a different metabolic pool from pituitary small GH. This may be newly synthesized small GH, not yet incorporated into granules, making it susceptible to non-specific binding to RNA or other proteins.Immunology and Cell Biology (1977) 55, 605–614; doi:10.1038/icb.1977.58