Myositis muscle contains antigen-matured B-cells and plasma cells. Myositis muscle biopsy specimens were examined for nodular collections of T-cells, B-cells, myeloid dendritic cells, plasma cells, and follicular dendritic cells. Immunoglobulin and B-cell-activating factor (BAFF) transcripts were quantitated. Laser-capture microdissection was used to isolate single plasma cells, and their immunoglobulin transcripts were sequenced. Dense inflammatory infiltrates contained histological elements of ectopic lymphoid tissue but not B-cell follicles. Immunoglobulin transcript sequence analysis demonstrated spatially distributed, clonally related B-cells and plasma cells, suggesting local maturation of B-cells into plasma cells in myositis muscle. Regions of dense cellular infiltrates in myositis muscle are sometimes areas of B-cell maturation into antibody-producing plasma cells. An atypical lymphoid histology, lacking concentrated collections of germinal-center-like B-cell follicles, is capable of antigen-stimulated clonal maturation of antibody-producing plasma cells.