The concept of soil receptiveness widely used for soil borne pathogens, is applied to the fungi forming vesicular-arbuscular endomycorrhizae. The authors propose a method for determining the mycorrhizal soil receptiveness (MSR) using leek, a highly mycotrophic plant, as a host for a bioassay. Under controlled conditions, populations of leek plants are grown in a soil inoculated with a range of inoculum levels. The inoculum consists of standardized root pieces infected with G. intraradices which are considered as propagules. The relationship between the percentage of plants forming mycorrhizae and the level of inoculum is used as a basis for determining the quantity of inoculum required to obtain mycorrhizae formation on 50% of the host plant population. The results are defined in terms of MSR unit, and are expressed as number of propagules corresponding to a MSR50 unit, or as MSR50 unit per propagule. This method is illustrated in a comparative study of four agricultural soils from France.