To the Editor: Although the development of monoclonal antibodies to human differentiation and tumor-associated antigens is still in its early stages, there is already a growing effort to use such antibodies for clinical purposes, both diagnostic and therapeutic. In a paper in the August 6 issue Cosimi et al. reported on infusing OKT3 monoclonal antibodies into renal-transplant recipients in order to suppress T-cell subsets.* Recent work in our laboratory has raised a substantial question about the safety of the use of mouse monoclonal antibodies in human beings. We have been developing monoclonal antibodies to human connective-tissue differentiation antigens appearing on. . No extract is available for articles shorter than 400 words.