Several investigators have suggested that gastrointestinal inflammation has a role in the pathogenesis of ankylosing spondylitis. To test this hypothesis markers of gastrointestinal immunostimulation, as manifested by serum IgA concentrations, were compared with serum markers of inflammation, as manifested by acute phase proteins. Serum samples from 45 unrelated Caucasian patients with ankylosing spondylitis (AS) were tested for correlation of serum IgA and six acute phase proteins: C reactive protein (CRP), alpha 1-antitrypsin, alpha 1-antichymotrypsin, caeruloplasmin, alpha 1-acid glycoprotein (AGP), and haptoglobin. Serum IgA was shown to be significantly positively correlated with four of these six acute phase proteins: CRP (r = 0.58, p less than 0.001), alpha 1-antitrypsin (r = 0.29, p less than 0.05), AGP (r = 0.61, p less than 0.01), and haptoglobin (r = 0.58, p less than 0.001), suggesting that gastrointestinal immunostimulation does have a role in the pathogenesis of inflammation in AS. In addition, the microheterogeneity of the pattern of glycosylation of AGP, expressed as reactivity coefficients, was examined. The AGP reactivity coefficient has been shown to increase in infection, remain the same in systemic lupus erythematosus, and decrease in rheumatoid arthritis. It was found that the AGP reactivity coefficient was significantly decreased in patients with AS as compared with healthy controls (p less than 0.006). As recent studies have indicated that patterns of glycosylation reflect intrahepatocellular biosynthetic processes induced by cytokines our data suggest that cytokine-hepatocellular mechanisms in AS may be similar to those occurring in rheumatoid arthritis, but different from those in systemic lupus erythematosus or infection.