We have performed experiments designed to evaluate the potential contribution of endotoxin contamination to lymphocyte reponses. Saline and EDTA extracts of 4 different strains of gram negative bacteria were examined for their capacity to initiate mitogenic responses in murine spleen cells. As compared to phenol extracts of these bacteria which contain primarily lipopolysaccharide-LPS, these saline and EDTA extracts were significantly less active in this assay. The mitogenic activity which was present was also manifest in spleen cells from the C3H/HeJ mouse, whereas phenol-extracted LPS preparations were inactive. In addition, mitogenic activity of saline and EDTA extracts was not blocked by polymyxin B, an agent known to abrogate LPS mediated responses. We conclude that LPS contamination may not normally be as significant a problem as had earlier been assumed. However, when endotoxin contamination is present, neither the use of C3H/HeJ spleen cells nor polymyxin B is an appropriate test to evaluate this possibility.