Sunflower (Helianthus animus L.) and mustard (Brassica hirta Moench.) plants were grown in autoclaved soil to which was added various components of soil inoculum. Use of inocula containing mycorrhizal propagules ultimately resulted in growth promotions which were related to improved phosphorus nutrition. Inoculation with soil containing mycorrhizal propagules caused an initial growth depression in comparison with plants receiving no inoculation, but inoculation with washed spores did not. For several reasons, the growth depression caused by inoculation with soil was not attributable to mycorrhizal infection. First, the growth depression was evident before mycorrhizal infections had become established. Second, the growth depression was also evident when either soil or soil sievings, both lacking mycorrhizal propagules, were used. Third, similar growth depressions were observed in mustard. The results suggest that careful selection of appropriate controls for mycorrhizal plants must be made, particularly if the emphasis is on early phases of plant growth. Use of non-sterile soil reduced the extent of mycorrhizal infection.