A structurally unusual, B-cell-derived malignant tumour is reported in a 24-year-old male. The gross autopsy findings were those of classical myelomatosis, with extensive and centrally located skeletal infiltrates and typical myeloma kidneys. The tumour cells were, however, morphologically very unusual. All elements were markedly pyroninophilic, indicating active protein synthesis, but typical plasma cell characteristics were lacking. The cells were ultrastructurally characterized by large and distended vacuoles, often with ribosomal granules attached, and containing an amorphous material. Virtually all tumour elements were found to be heavily loaded with IgG(kappa) in immunofluorescence studies. IgG paraprotein was also found in serum, and kappa light chains in the urine. It is concluded that the tumour represents an intermediate form between multiple myeloma and a tumour originating from the large pyroninophilic, immunoglobulin-secreting lymphocytes.