The effect of enteric coating of aspirin tablets on the gastrointestinal blood loss associated with high dose aspirin therapy was investigated in 12 patients with rheumatoid arthritis. Occult blood loss was measured after labelling the patients' red blood cells with Cr51. Three salicylate preparations were used: enteric coated tablets of aspirin ("Rhusal", G.P. Laboratories, 7 x 650 mg per day), uncoated tablet cores of aspirin from the same batch (7 x 650 mg per day) and enteric coated tablets of sodium salicylate (7 x 600 mg and 1 x 300 mg per day). Daily blood loss during a salicylate-free period was (0.7 +/- 0.15 ml, mean +/- SE). Blood loss was significantly increased during dosage with all three salicylate preparations. Daily blood loss during dosage with the uncoated tablets of aspirin (5.3 +/- 0.3 ml) was significantly greater than during dosage with the enteric coated tablets of aspirin (2.3 +/- 0.3 ml) and enteric coated tablets of sodium salicylate (2.1 +/- 0.4 ml). The bioavailability of the salicylate preparations was studied in seven of the 12 patients. Mean plasma salicylate concentration two hours after the second daily dose during dosage with the enteric coated tablets of aspirin was 118 +/- 15 microgram/ml compared to 131 +/- 16 microgram/ml during dosage with the uncoated tablets. Urinary recoveries of the daily dosage of aspirin in the two formulations were also similar.