Immunoprecipitation followed by Western blotting of supernatants of HUVECs and THP-1 cells. HUVECs and THP-1 cells were stimulated with 0.1 μg/mL LPS for 4 hours and 1.5 mL of the supernatant was immunoprecipitated with a polyclonal anti–IL-1β antiserum, followed by Western blotting as described in the Materials and Methods. No signal in any lane could be observed at 31 kD, the size of the IL-1β precursor molecule, whereas at 18 kD a signal can be seen in HUVECs and stimulated THP-1 cells. At 23 kD, the lower band of Ig can be observed. The position of the Ig heavy chain is 43 kD.