Calprotectin, the L1 leucocyte protein, is found in large quantities in the cytosol of granulocytes and monocytes. Plasma calprotectin levels are increased in infections, malignant tumours, vascular insults and various other pathogenic conditions. The authors have investigated plasma calprotectin and ANCA levels in 27 patients with endogenous posterior uveitis (EPU) and six healthy volunteers. Compared to the control values, the mean levels of plasma calprotectin were raised in patients with active uveitis (p<0.005 (ANOVA)). Raised serum ANCA titres, which are also associated with neutrophil activation, were also detected in some patients with EPU but the level of ANCA did not correlate with that of calprotectin. The authors suggest that measurement of plasma calprotectin may be a sensitive indicator of disease activity in patients with endogenous posterior uveitis.