Effects of LPS preincubation on intracellular IL-12 (p40/p70) production by human monocytes. Human monocytes were stimulated with 300 U/mL IFN-γ and 50 ng/mL LPS for 24 hours. Preincubation was performed as indicated in Materials and Methods using 50 and 10 ng/mL LPS, respectively. The percentages of monocytes positive for IL-12 (p40/p70) or IL-6, as detected by flow cytometric measurement, are shown. Results are given as mean ± standard error of mean (SEM) of 13 different donors. The downmodulation of IL-12 by preincubation with LPS was significant (P < .0001 when 50 ng/mL LPS were used, P < .005 for 10 ng/mL LPS).