Leukotriene B4 (LTB4) is an activator of white blood cells (WBC) and it has been suggested that its inhibition may be useful in rheumatoid arthritis (RA). Its production by peripheral WBC has not yet been investigated. We measured LTB4 production in 105 patients with RA and compared it with 59 matched controls. C-reactive protein (CRP) and ESR were measured in 90 patients and correlated with LTB4 values. Ten millilitres of blood were drawn. Separation was undertaken to obtain polymorphonuclear leukocytes (PMN) which were stimulated with calcium ionophore, and the supernatant was frozen for radioimmunoassay of LTB4. Results show that RA patients produce significantly higher levels of LTB4. It has been suggested that blockage of the cyclo-oxygenase enzyme by non-steroid anti-inflammatory drugs (NSAID) leads to increased production of LT via the lipoxygenase enzyme. Twenty-one patients not taking NSAID were compared with 84 on therapy. There was no significant difference. A linear regression was used to obtain Pearson's correlation coefficients. With LTB4 and CRP, r = 0.3 (p < 0.003). With LTB4 and ESR, r = 0.25 (p < 0.02). Low but significant correlations with CRP and ESR were obtained.