Serum and plasma normally inhibit the responsiveness of the indicator cells used in bioassays for interleukin-1 (IL-1). A precipitation step with 12% polyethylene glycol (PEG) was found to remove material responsible for such inhibition. The IL-1 bioassay could be performed in the presence of a 3% concentration of PEG and recovery of added natural IL-1 from plasma or serum was essentially complete.