We studied the suitability of in vitro dialysis with polyacrylonitrile (PAN) membranes to remove small amounts of interleukin-1 beta (IL-1 beta) and tumor necrosis factor (TNF) from plasma to be used as diluent for the standards in radioimmunoassays (RIA). Incubation of PAN membrane fragments with radiolabeled IL-1 beta or TNF yielded a significant binding of both cytokines to the membrane (percentage of membrane-bound cytokine after incubation in saline or plasma was 14-17% and 23-46%, respectively). Dialysis of plasma (containing radiolabeled cytokine) against plasma (initially devoid of cytokine) resulted in a binding percentage of IL-1 beta and TNF to the PAN membranes of 44 and 28%, respectively. When plasma was dialyzed against saline the percentage of membrane-bound IL-1 beta and TNF was 63 and 37%, respectively. After dialysis of plasma against either plasma or saline the percentage IL-1 beta recovered from the dialysate was approximately 16% in contrast with 1-2% TNF. The results confirm the capacity of in vitro dialysis with PAN membranes to remove IL-1 beta and to a lesser extent TNF from plasma. Removal is most marked in the first minutes of dialysis (suggesting saturation of the membrane) and less effective for TNF due to its low diffusion across the membrane.