The relationship was examined between the density of vesicular-arbuscular (VA) mycorrhizal inoculum and the growth of Phleum pratense L. and Agropyron trachycaulum (Link) Malte. The most probable number method (MPN) was used with combinations of host, VA mycorrhizal inoculum and a series of dilutions of field soil. There was a positive correlation between the proportion of field soil and the amount of mycorrhizal colonization. The numbers of tillers and leaves, shoot and root dry weights, and root:shoot ratios were negatively affected by increasing proportions of field soil inoculum. The addition of fertilizer alleviated the effects of increasing proportions of field soil on plant growth and reduced the amount of colonisation by mycorrhizal fungi. The relationship between increasing proportions of field soil and plant growth was not species-specific. We conclude that there was an interaction between the inoculum density of VA mycorrhizal fungi and soil nutrient availability which affected plant growth.