The secretions of interleukin 1 (IL-1), tumour necrosis factor alpha (TNF), and prostaglandin E2 (PGE2) of low-dose E. coli lipopolysaccharide (LPS)-stimulated human monocytes (M phi) were investigated in an endotoxin (ET)-free milieu (less than 1.6 pg LPS/ml). Human M phi cultures from nine healthy men were stimulated with 0, 12.5-500, and 250,000 pg LPS/ml as measured by a very sensitive Limulus test. The IL-1 activity was tested by the mouse costimulatory thymocyte (LAF) assay, which was thoroughly standardized and characterized (interassay variation 22-24%, intra-assay variation 3-7%). Spontaneous M phi secretions of IL-1, TNF, and PGE2 were negligible, but 12.5 pg LPS/ml significantly stimulated the secretions of these M phi products and the monokine responses to 500 and 250,000 pg LPS/ml were almost in the same range. It was demonstrated that the secretions of IL-1-TNF and TNF-PGE2 were strongly correlated. Pronounced interindividual differences in LPS responsiveness were demonstrated, and two low-responders, one of whom was HLA-DR1,2-positive, were identified. Three first-degree relatives of the DR1,2-positive low-responder had similar low responses. Furthermore, M phi cultures were prepared weekly for 4 weeks from four HLA-DR different men and the only DR2,2 homozygous individual had low monokine responses. In conclusion, stable interindividual differences in in vitro monokine and PGE2 secretions of LPS-stimulated M phi were demonstrated. It is suggested that HLA-DR2-positive individuals may be low responders.