Pig digestive tract mucins have often been used as model mucins for studying mucin structure, function and metabolism. In the present study pig gastric mucin and pig colonic mucin in the subunit form have been characterised and compared. Following Sepharose 4B or 2B-CL gel chromatography, the mucin eluant fractions were assayed colorimetrically by both the periodic acid-Schiff and the Alcian blue binding assays. Subunit colonic mucin eluted as a single unimodel peak that was easily detected by both assays. In contrast, subunit gastric mucin gave a peak primarily detected by periodic acid-Schiff that was overlapped by, but partially separated from, another peak primarily detected by Alcian blue. Subunit gastric mucin was separated into two periodic acid-Schiff staining spots when electrophoresed on cellulose acetate. Cetylpyridinium chloride (CPC) was able to precipitate only about half the subunit gastric mucin. The CPC-precipitable subunit gastric mucin corresponded to the faster running spot on electrophoresis, and the subunit gastric mucin in the CPC supernatant (which may have more than one subunit mucin type) to the slower spot(s). The former had a higher sulphate content and stained with Alcian blue. The latter had a lower sulphate content and showed very little Alcian blue reactivity. These results indicate that subunit pig gastric mucin is heterogeneous with respect to both size and charge. The differences between the types may be important in biological and physiochemical behaviour of gastric mucin. It seems likely that different laboratories may have worked on one or other of the pig gastric mucin types or a mixture, depending on the preparation method.