Plasma interleukin-1 (IL-1) activity was measured in 7 stable ESRD patients on regular hemodialysis for no less than 5 months. Predialysis levels were significantly raised compared to 8 normal control subjects. During hemodialysis with four different membranes, plasma IL-1 activity rose with Cuprophan and Hemophan and was unchanged or reduced with Gambrane and Polysulfon. In spite of these differences, body temperature rose in all forms of hemodialysis. Factors responsible for the predialysis elevation included the absence of renal function and/or the repeated stimulus of human blood monocytes by hemodialysis. In view of the uniform increase of body temperature during hemodialysis, the differences in changes of plasma IL-1 activity observed with the various membranes may not be caused by a variable stimulation of monocytes but rather by the presence or absence of the membrane's ability to remove and/or absorb IL-1. Thus, the consequences of monocyte hemodialysis stimulation may be obtained locally, even in the presence of unchanged or reduced plasma IL-1 activity.