We have investigated the expression of mRNA for interleukin 6 (IL-6) in cells infiltrating 12 epiretinal membranes, and the presence of biologically active IL-6 in vitreous humour available from five corresponding eyes. The results showed that nine of the 12 membranes (75%) contained cells expressing mRNA for this cytokine. Although in two of the specimens pigmented cells were identified as some of the cells expressing mRNA for IL-6, we did not identify the nature of IL-6 mRNA-producing cells infiltrating the membranes. Interestingly, two vitreous samples from eyes whose membranes did not contain cells with mRNA for IL-6 exhibited significant concentrations of IL-6 (315 and 28 micrograms/ml). Parallel study of mRNA for IL-6 in PVR biopsies and of IL-6 levels in corresponding vitreous may indicate how cytokine-mediated pathways of inflammation are involved in the pathogenesis of epiretinal membrane formation.