NITROGEN METABOLISM IN P. GRISEOFULVUM 289 this fraction. Incorporation into glutamic acid was sufficient to account for the synthesis of at least 85% of the total mycelial ao-amino nitrogen content. The redistribution of label observed after the exhaustion of exogenous nitrogen confirmed previous non-isotopic evidence that a rapid turnover of insoluble nitrogen, involving breakdown to the amino acid level, occurred during nitrogen starvation. Extracellular organic nitrogen released after exhaustion of ammonia arose from the breakdown of insoluble mycelial material. Thanks are due to Mr ID. H. W. Scott for skilled technical assistance in the work describecL in this and the two preceding papers. I am also grateful to Mr R. G. Harrison for carrying out determinations with the mass spectrometer, and to Mr A. F. Henson for help in interpreting the isotopic data.

Graft-versus-host disease is one of the major problems in clinical bone marrow transplantation. Many experiments in animals have shown that it could be greatly reduced if mature T lymphocytes were removed from the donor marrow. Here we describe a new rat monoclonal antibody, CAMPATH 1, which is suitable for depleting lymphocytes from human marrow grafts. CAMPATH 1 is an IgM that fixes human complement. It binds to both T and B lymphocytes and some monocytes but not to other hemopoietic cells. When peripheral blood mononuclear cells were treated with CAMPATH 1 and complement, more than 99% of lymphocytes were killed and viable T cells could no longer be detected. Under these conditions, in vitro multipotential erythroid and myeloid colony-forming cells were unaffected. As well as being used for in vitro treatment of bone marrow to remove T cells, CAMPATH 1 could potentially be applied to other experimental and clinical situations where depletion of lymphoid cells is required, including serotherapy to achieve immunosuppression for organ transplants or to treat lymphocytic leukemias.

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