Effects of mycorrhizal infection on the growth of Trifolium subterraneum have been studied by comparing plants grown in sterilized soil/sand mixtures with plants grown in untreated soil/sand mixtures. The original soil contained a high level of endomycorrhizal inoculum. The aim was to consider whether this approach is valid, bearing in mind the possible effects of sterilization (γ-irradiation or autoclaving) on soil properties. Plants in the untreated soil/sand mixtures became significantly larger than plants in the sterilized soil/sand mixtures about 4 weeks after planting. This response was associated with rapid infection by vesicular-arbuscular mycorrhizal fungi; it still occurred when nitrate replaced N2 as the nitrogen source for the plants. Addition of soluble phosphate enhanced the growth of the non-mycorrhizal plants to a level similar to that of the mycorrhizal plants. Mixing sterilized and untreated soil did not depress mycorrhizal infection or growth. These results indicate that the growth difference was a classic mycorrhizal response, and did not reflect toxic effects of soil sterilization. The growth of Brassica oleracea (broccoli), which does not form mycorrhizas, was better on sterilized soil.