Tumors are described which have been produced by subcutaneous (s.c.) implantation of nickel subsulfide (Ni3S2) in 00 gelatin capsules in the left axillary region of Fischer 344 rats. This is in contradistinction to previous accounts where intramuscular (i.m.) or, in a few cases, subcapsular testicular implantation sites have been used. The advantages of the s.c. route are discussed, including the removal of the difficulties of distinguishing degenerating and regenerating striated muscle cells from tumor cells. The s.c. induced tumors were transplanted as far as the F6 generation, in some cases. Cells from tumor transplanted as far as the F3 generation were grown in tissue culture and then reimplanted s.c. Cytology of the tumors remained remarkably constant in all tumors, even after serial transplants and culture. A characteristic storiform pattern, with two major cell types -myofibroblasts and histiocyte-like cells, was seen. The question of pluripotential cell origin is discussed. The striking similarity of appearance to human malignant fibrous histiocytoma is noted; however, in view of the somewhat controversial origin of the human tumor, the simple designation, "nickel-induced malignant tumor of pluripotential origin", is preferred. The use of this model with its demonstrated ability to reproduce characteristic appearances seems to be valuable for the study of soft tissue sarcomas.