The cellular composition of the dermal infiltrates of eleven patients with a cutaneous B-cell lymphoma (four centroblastic lymphomas, two centroblastic/centrocytic lymphomas and five immunocytomas) was investigated. The distribution of both the neoplastic and the non-neoplastic cells (reactive T cells, macrophages and dendritic reticulum cells) in primary and secondary cutaneous B-cell lymphomas was very similar to that of B-cell lymphomas of the same type in lymph nodes. Reactive T cells and dendritic reticulum cells were only occasionally found in centroblastic lymphoma, but were very numerous in centroblastic/centrocytic lymphoma. The large majority of these T cells in centroblastic/centrocytic lymphoma showed the phenotype of activated T-helper cells (Leu-I+, Leu-3a+, OKT4+, HLA-DR+). In immunocytomas many T cells reactive with Leu-I, Leu-3a, and OKT4 but not with anti-HLA-DR antiserum, and varying numbers of dendritic reticulum cells were found. Since B-cell lymphomas in lymph nodes are the neoplastic counterparts of B-cell reactions which take place after antigenic stimulation in the different lymph node compartments, our results suggest that cutaneous B-cell lymphomas may be the malignant counterparts of similar B-cell reactions in the skin.