Polarisation microscopy of material obtained by fine needle biopsy of subcutaneous tissue and stained with Congo red is a simple and reliable method for the diagnosis of systemic amyloidosis. It cannot, however, be used to differentiate histologically between different forms of amyloidosis. In the present study extracts of material obtained by fine needle biopsy of subcutaneous fat tissue from 13 patients were examined by double immunodiffusion with an antiserum against protein AA, a unique protein which forms a major part of the fibrils in secondary amyloidosis. Five of the patients showed amyloid deposits round the fat cells by conventional microscopy. In 3 of these, all with rheumatoid arthritis, protein AA was detected. Eight patients without amyloidosis and 2 with myelomatosis and amyloidosis showed no reaction with antiprotein AA antiserum. Thus the material obtained by fine needle biopsy of subcutaneous tissue could be used not only for the histological diagnosis of amyloidosis but also for a classification of systemic amyloidosis into secondary or primary based on the type of amyloid fibril protein involved.