Cytoplasmic immunoglobulins in human bone marrow plasma cells and lymphoid cells were characterized by direct immunofluorescence with fluorochrome-labelled reagents specific for immunoglobulin heavy and light chains. The percentage distribution of cells containing IgA, IgG or IgM and χ- or λ-immunoglobulins was determined in bone marrow samples from 168 immunologically normal individuals, in 11 patients with polyclonal increase of bone marrow plasma cells and in 80 patients with benign or malignant monoclonal gammopathies. A clear differentiation between monoclonal and polyclonal cell populations could be obtained in all cases.