The effects of fungal spores and bacterial lipopolysaccharide (LPS) on superoxide anion production and guinea-pig bronchial alveolar lavage (BAL) cell activation was studied to determine the link between fungal spores, gram negative bacteria and their endotoxins to organic dust toxic syndrome (ODTS). BAL cells from English-Short- Hair-Guinea-pigs were incubated with LPS and/or fungal spores from Aspergillus-candidus, Aspergillus-terreus, Aspergillus-niger, Aspergillus-fumigatus, Eurotima-amstelodami, Penicillium-spinulosum and Cladosporium-cladosporioides, isolated from material associated with ODTS. Superoxide anion release was measured spectrophotometrically at a wavelength of 570 nanometers through the reduction of cytochrome-c using a 96 well plate reader. Spores from each fungus were able to effect an increase in superoxide anion production. The amount of production varied with each species. The study showed that LPS used alone did little to induce BAL cell activation. However, pretreating cells with LPS and then exposing them to fungal spores resulted in an increase in superoxide anion production which exceeded that of either the spores or LPS used separately. Thirty minutes was found to be the best pretreatment interval. The authors conclude that bacterial LPS strongly stimulates cell activation by fungal spores.