We have demonstrated that lymphoid leukaemia L 1210 cells treated with cyclophosphamide or ifosfamide were immunogenic for semisyngeneic CD2F1 mice and that they effectively prevented the development of the later inoculated leukaemia L 1210 cells. Non-dividing and immunogenic L 1210 cells were only obtained when an appropriate dose of cyclophosphamide was applied. These cells lost their immunogenicity after killing by repeated freezing and thawing or after fixing with glutaraldehyde. Non-dividing immunogenic L 1210 cells treated with cyclophosphamide possessed the histocompatibility and tumour-associated transplantation antigens and had the ability to synthesize proteins, the RNA and partially the DNA.