It is generally agreed that the demonstration of L. E. cells in the bone marrow or blood is a highly specific test for systemic lupus erythematosus. Aside from its value in diagnosis, the test is proving useful to the clinician in broadening his understanding of many hitherto puzzling features of the natural history of the disease, its often protracted and variable course, as well as its protean symptomatology. One of the most satisfactory laboratory procedures for the demonstration of L. E. cells is by means of the two-hour clot technique on peripheral blood. This so-called L. E. clot test has been in use in the laboratories of the Mayo Clinic since the fall of 1951 as a screening procedure in cases in which the clinician had some reason to suspect the presence of lupus erythematosus. In order to evaluate the L. E. clot test more