Inbred rats were hyper-immunised against a syngeneic transplanted mammary carcinoma, Sp4, which originally arose spontaneously in a breeding female. Spleen cells from Sp4-immune rats were fused with P3-NSI-Ag4 mouse myeloma cells, and supernatants from the hybrid progeny were screened for antibody reacting with Sp4 cells using an isotopic antiglobulin assay. One hybrid culture was reactive with Sp4 cells but not against other target cells, and this hybrid was cloned in soft agar. Many Sp4-positive clones were isolated and one (Sp4/A4) was chosen for detailed study. Clone Sp4/A4 was positive only with Sp4 cells and failed to react with cells of other tumours or normal tissues. It contained much higher titres of antibody than detected in immune sera, and it is suggested that this approach offers a practical method for producing highly active tumour-specific antibodies.