It has been proposed that monocyte activation during hemodialysis involves the release of interleukin-1 (IL-1) and related cytokines. In this review, the biological properties of recombinant human IL-1 are discussed with particular emphasis on hemodialysis. Models of monocyte activation which likely occur during hemodialysis are presented. We conclude that pyrogens, solutes, complement components and the physical nature of the dialysis membrane itself contribute to monocyte activation and cytokine release.