Dose response of TNF production by THP-1 cells stimulated with Stx1. Differentiated THP-1 cells were incubated with the indicated concentrations of Stx1 or LPS for 12 hours. Cell-free culture supernatants were collected and TNF bioactivity was quantitated by L929 assay as outlined in the Materials and Methods. Data are expressed as the mean (in picograms per milliliter) ± SEM of three separate experiments.