Certain probiotic bacteria show anti-inflammatory activity after being heat killed, whereas others do not, suggesting that the gastrointestinal environment may alter the activity of probiotic bacteria. Occasionally, probiotics are provided when a patient is also being treated with oral antibiotics; this may have an effect on the probiotic activity. We hypothesized that Lactobacillus rhamnosus GG (LGG) are capable of downregulating tumor necrosis factor-alpha (TNFalpha)-induced interleukin (IL)-8 production under all 3 of these conditions, and that LGG act through the nuclear factor kappaB (NFkappaB)/inhibitor of kappaB (IkappaB) pathway. Caco-2 cells were treated with live or heat-killed LGG in doses ranging from 10(4) to 10(10) cfu/L, in the presence or absence of antibiotics and TNFalpha in the media. TNFalpha-induced production of IL-8 by Caco-2 cells was modulated by LGG under all 3 conditions. However, higher doses of live LGG without TNFalpha in the presence or absence of antibiotics in vitro induced the production of IL-8 (P = 0.001). Heat-killed LGG also blunted the TNFalpha-induced IL-8 production (P < 0.01), but by itself did not increase IL-8 production at higher doses as markedly as live LGG (P < 0.05). LGG reduced the TNFalpha-induced NFkappaB translocation to the nucleus and lessened the decrease in IkappaB in the cytoplasm (P < 0.05). LGG reduced TNFalpha-induced IL-8 production by affecting the NFkappaB/IkappaB pathway in Caco-2 cells. High doses of live LGG markedly increased IL-8 production, but heat-killed LGG caused only a slight increase in IL-8. Thus, heat-killed LGG may effectively ameliorate inflammation with a lower potential than live LGG at high doses to cause inflammation.