The natural killer (NK) cell activity of unfractionated peripheral blood and synovial fluid mononuclear cells from patients with inflammatory joint disease was measured in a short-term assay using the human tumour cell line, K562, as the target. The mean values for peripheral blood NK activity of the various groups (controls, rheumatoid arthritis (RA), ankylosing spondylitis (AS) and psoriatic arthritis (PsA] were similar although the rheumatoid group showed the widest range. However, the NK activity of PsA patients (23.74 +/- 10.14) was significantly lower than that of the controls (31.63 +/- 10.8, 0.05 greater than P greater than 0.01). Almost without exception, NK activity was found to be considerably lower in synovial fluid than in paired blood samples (p less than 0.01).