The expression of Fc mu receptors was investigated using spleen cells from (LOU/M/Wsl X CFY) F1 rats bearing the IR202 tumour, an IgM-producing immunocytoma. It was found that the progressively growing solid tumour was accompanied by an extraordinary expansion of host splenocytes with Fc mu receptors. These cells were nylon-wool non-adherent, Thy-1-positive and sIg-negative: features they have in common with T lymphocytes. In addition, the expansion of Fc mu receptor-bearing T cells in IR202 immunocytoma is correlated to the high level of serum IgM. These observations provide further insight into the mechanism of isotype-specific T-lymphocyte Fc receptor expression, and identify a potential model with which to analyse the immunoregulatory role of Fc mu receptor-bearing T cells.