There are several reports on the occurrence of immunoglobulin-producing B-cell lymphomas in Sezary's syndrome and mycosis fungoides, which may be the consequence of the helper activity of the neoplastic T-cells. Therefore we investigated skin biopsies of 50 patients with mycosis fungoides regarding the presence of plasma cells and their immunoglobulin profile. Nine of these patients had plasma cell nests, most frequently located at the lower edge of the infiltrate. IgE was detectable consistently, and IgG, IgM and IgA could also be demonstrated in the majority of these cases; kappa- and lambda-chains were present in equal amounts. Our results demonstrate polyclonal activation of plasma cells in a subgroup of mycosis fungoides patients.