Three reticulum cell sarcoma lines (LA1, LA6, and LA8) have been established from SJL/J spontaneous "Hodgkin's-like" reticulum cell sarcoma. All cultures are lymphoid with blast-like morphology by light and electron microscopy and produce the type B neoplasm (Dunn classification) when injected into young SJL/J mice. Identification of these tumors as lymphocytic and monocytic was investigated by surface markers, histochemical staining, and phagocytic function. LA6 and LA8 bear receptors for the Fc portion of immunoglobulin complement receptors; Thy 1.2 antigens and surface immunoglobulin were not detected on any of the three lines. No lines were able to synthesize immunoglobulin or phagocytose Degalon beads. Histochemical staining was presumptive of lymphocytes or lymphoblasts with slight differences between the lines. Although a T- or B-cell classification cannot definitively be made for these tumors lines because of their lack of specific markers, the results are consistent with an immature B-cell origin for the SJL/J reticulum cell sarcoma.