SUMMARY Metatarsophalangeal and metacarpophalangeal joints from 3 patients with rheumatoid arthritis were investigated electron microscopically with regard to the occurrence of polymorphonuclear granulocytes (PMNs) at the pannus-cartilage junction. In all 3 cases PMNs could be detected at the junction and within the cartilaginous matrix. PMN cytoplasmic processes surrounded collagenous islands in the cartilage. From the morphological findings it is deduced that PMNs are cells capable of destroying cartilage in inflammatory joint diseases, in particular in rheumatoid arthritis.Using enzyme histochemical and immune histological investigations we recently demonstrated the appearance of polymorphonuclear granulocytes (PMNs) at the pannus-cartilage junction in rheumatoid joints.1-4 As some doubt remains about the specificity of the naphthol-AS-D-chloroacetate esterase reaction5 for the demonstration of PMNs with regard to the content of this enzyme in monocytes,fi we tried to characterise PMNs at the pannuscartilage junction by electron microscopy. MAterial and methodsSurgically removed metatarsophalangeal and metacarpophalangeal joints were fixed in buffered formalin. After decalcification in Versen (Titriplex III) the specimens were bisected. One half of the tissue was processed by the routine paraffin method, stained with haematoxylin-eosin and the naphthol-AS-D-chloroacetate esterase reaction.5 The second half was stored in formalin. Joints from 3 patients ( Table 1) with accumulations of PMNs at the pannuscartilage junction, proved by light microscopy, were chosen for electron microscopy. From areas corresponding to the histological PMN-rich foci small pieces of the articular cartilage with adherent pannus tissue were dissected and rinsed in water. Thereafter