Rhizobial Nod-factors are N-acylated oligomers of N-acetyl-D-glucosamine carrying a variety of substituents. At pico- to nano-molar concentrations, they provoke deformation and curling of nodulation competent root-hairs. Higher concentrations (10⁻⁷ to 10⁻⁶ M) elicit preinfection structures, allow Nod⁻ rhizobia to enter legume roots and to fix nitrogen within nodules. Evidence is accumulating that nodulation capacity is controlled both by specific Nod-factors as well as by Nod-factor concentrations. As an example, root-hairs of Macroptilium atropurpureum show identical "shepherd's crook" type curling when challenged with optimal (≈ 10⁻⁹ M) concentrations of Nod-factors of Bradyrhizobium japonicum and Rhizobium sp. NGR234 (which are Nod⁺) or with Nod-factors of R. leguminosarum and R. meliloti (both bacteria are incapable of nodulating Macroptilium). Similarly, conjugation of the nodD1 gene of NGR234 into R. meliloti allows nodulation of M. atropurpureum. We have used alkanisation of the tomato cell growth medium as a quantitative bioassay for Nod-factors. Upon induction with flavonoids, wild-type Rhizobium sp. NGR234 produces abundant Nod-factors, which may partly explain its extremely broad host-range. Nod-factors are produced throughout the log-phase of growth, while mutations in nodSU drastically reduce Nod-factor synthesis. Good correlations exist between the levels of Nod-factors and the ability to nodulate specific legumes.