According to present-day views, the LE-cell arises through the action of the so-called LE-factor – localised in the gamma-globulin fraction – on nuclear material which changes in such a way that it undergoes phagocytosis, usually by granulocytes, sometimes by monocytes, and occasionally by other blood cells. For the development of this phenomenon the presence of the LE-factor, nuclear material, and phagocytosis is therefore necessary. Since Hargraves' original publication in 1948, many techniques have been described. These can be divided into direct and indirect methods, with or without the addition of anticoagulants. In the direct test the patient’s whole blood is used, supplying the LE-factor, the nuclear substance and the phagocytes. In the indirect test the patient’s serum or plasma is added to nuclear substance and phagocytes present in human or animal blood. This so-called leucocyte-donor should, of course, not be suffering from systemic lupus erythematosus. Suksta and Conley showed that a certain incubation period is necessary for the formation of the LE-cells. According to Haserick this need for an incubation period can account for the difficulties which were first met with in confirming Hargraves' findings. He applied the heparin concentration method for bone-marrow smears according to Schleicher and Sharp. This method implies an incubation period which is absent when direct bone-marrow smears are made. It was initially thought that heparin or other anticoagulants played a role in the LE-cell formation. Eppes and Ludovic, however, were also able to demonstrate LE-cells in the huffy coat of defibra- nated and unclotted blood using siliconed glassware. Lee described a method with clotted blood from which the huffy coat was removed after fragmentation. This method was more sensitive than the techniques described up till then. Zimmer and Hargraves increased the sensitivity by doubling the concentration of the huffy coat obtained from the fragmented clot. They were of the opinion that a relationship existed between the clotting of blood and the increased production of LE-cells. From their experiments Zinkham and Conley concluded that a mechanical factor is responsible for the increased sensitivity and indicated the importance of traumatised leucocytes as a readily available substrate for the LE-factor. There are also several possibilities for the indirect test. Hargraves added LE-serum to human bone marrow, Berman et al. to animal bone marrow, Barnes et al. used patient’s serum in combination with a huffy coat from heparinized or defibrinated peripheral human blood, and Carrera et al. with a huffy coat from heparinized peripheral animal blood. An interesting method has been given by Snapper, whereby blood from a patient is added to dried nuclear material derived from normal leucocytes. The published indirect methods have the disadvantage of being relatively insensitive. A further analysis of the LE-phenomenon necessitated a more sensitive indirect test. Therefore, an indirect method was developed in our laboratory, based on the Zimmer and Hargraves technique, in which the LE-factor is incubated with a finely fragmented blood clot of a leucocyte donor; from this a doubly concentrated huffy coat is obtained, which is then used for making smears. The description of this technique and the results obtained form the subject of this paper.