A group of 12 large-cell lymphomas, seen in the period January 1983 to January 1985 and then tentatively classified as histiocytic sarcomas/true histiocytic lymphomas (i.e. non B-, non T-cell lymphomas, with expression of some histiocytic markers) were restudied phenotypically with a much more extensive range of monoclonal and polyclonal antibodies. They were also genotypically analyzed with respect to rearrangements of genes coding for the immunoglobulin (Ig) heavy and light chains and the gamma and beta chains of the T-cell receptor (TCR). The combined results suggest that these "histiocytic sarcomas" represent a heterogeneous group of large cell lymphomas often with unexpected rearrangements (or lack of rearrangements) of either Ig and TCR genes and phenotypically with coexpression of "histiocytic" and B- or T-cell markers. Such coexpression may be regarded as a warning signal to expect unusual genotype. True histiocytic tumours apparently do exist, but are rare (one case in this study), and their diagnosis is very difficult.