In an earlier study, the mean plasma carcinoembryonic antigen (CEA) level of patients with seropositive rheumatoid arthritis (RA) was found to be significantly higher than that of normal, control subjects. Levels of CEA in patients with seronegative RA and ankylosing spondylitis, however, did not differ from normal. In this study it was shown that the mean CEA level of 16 patients with Sjögren's syndrome was also significantly higher than normal (P equals 0.001), whereas in a group of 23 children with Still's disease the mean level fell within the normal adult range. Despite the apparent association between rheumatoid factor and raised plasma CEA, no correlation was found between titre of rheumatoid factor and CEA levels. Gel filtration studies indicated that the CEA in rheumatoid arthritis was of a similar molecular weight to that found in cancer of the colon and that there was minimal contribution by the known cross-reacting antigen CCEA2. The mean CEA level in rheumatoid synovial fluids was found to be significantly higher than in osteoarthrotic fluids. A preliminary study has also shown that CEA can be extracted from rheumatoid synovial membranes but was not detected in a normal synovium, further indicating that the source of this antigen in RA may be the inflamed synovium.