We sought the presence of lymphocytotoxic antibodies in the serum of 67 children with juvenile rheumatoid arthritis. We demonstrated in the serum of a large number of children (more than 50 %) antibodies reacting mainly with the cells not forming spontaneously rosettes with sheep red cells and/or all the mononuclear cells of the peripheral blood of normal subjects. These antibodies which are more active at 4 degrees C than at 20 degrees C are absorbable on red cells, and/or the platelet pool and present the characteristics of cold lymphocytotoxins. The presence of these antibodies was not correlated with either the clinical form of juvenile rheumatoid arthritis, nor with the duration of the course, nor with the treatment. Generally speaking, such antibodies are found during diseases where regulation of the immune response is modified.