C57BL/10ScCr (Cr) mice carry a deletion of the Toll-like receptor 4 (tlr4) gene (i.e. they are tlr40/0) and are thus refractory to LPS effects. Insertion of wild-type tlr4 transgene into the tlr40/0 Cr germ line endowed LPS susceptibility in the two transgenic lines created, indicating that TLR4 is the only limiting factor for LPS responsiveness in Cr mice. The absolute levels of tlr4 mRNA expressed by the heterozygous transgenic (tlr4Tr/0), wild-type C57BL/10ScSn (Sn) (tlr4+/+) and heterozygous F1 (Sn × Cr) (tlr4+/0) mice varied markedly. However, the pattern of distribution of expression in the different organs was the same in all strains. In different biological assays (B cell mitogenicity, cytokine induction and lethal toxicity) the degree of LPS response obtained in the different strains of mice correlated with the levels of tlr4 mRNA expression. In macrophages, investigation of the LPS-induced cytokine (IL-6) response revealed a linear relationship between the response and the logarithm of TLR4–MD-2 levels.