Cyclosporin (CsA) is a potent immunosuppressive drug widely used in organ transplantation. We transplanted fresh surgical samples from human solid malignant tumours into 45 CsA-immunosuppressed rats. Eight out of nine tumour types grew and remained viable for 5 weeks or more in at least two of the transplanted rats. In 29 rats (64%) a distinct growth of primary human tumours was recorded. Five malignancies (intestinal-type gastric carcinoma, adenocarcinoma of the lung, lymph node metastasis of a testicular teratocarcinoma, soft tissue malignant fibrous histiocytoma (MFH), and small-cell sarcoma) showed invasive and progressive growth. In all five cases the largest tumours were 0.9 cm or over in diameter when the rats were killed 5-9 weeks after transplantation. In three cases (adenocarcinoma of the colon, hypernephroma, and a second MFH) the growth of the implants under the kidney capsule was slow, but small living tumour transplants were still found 3-6 weeks later. In every case the microscopic morphology of the xenograft tumour was identical with the original tumour. In two cases the primary xenografts (teratocarcinoma and small-cell sarcoma) were retransplanted into 11 CsA-immunosuppressed rats. In both types the second passage tumours grew, and the take-off and growth rates were comparable to the primary xenografts. Cyclosporin-treated laboratory rats are an alternative to immunodeficient nude and SCID mice for growing fresh human tumour transplants in vivo. Although a few infections were encountered, most of the rats survived the CsA treatment well for up to 2 months.