A soil-sandwich bioassay was used to determine the influence of temperature, moisture, form of nitrogen, and the soil microflora on the saprophytic growth through soil of an isolate of Trichodel ma koningii. Incubation temperature affected the saprophytic growth of the fungus in soil over the range tested. Saprophytic growth of the isolate in sterile soil increased with incubation temperature from 5 degrees C to the optimum temperature 25 degrees C; there was no saprophytic growth at 30 degrees C. Saprophytic growth also increased with soil moisture content in sterile soil, with a growth optimum at 70% soil water holding capacity (WHC). There was little saprophytic growth at soil moisture contents below 20% WHC. Nitrogen added as ammonium sulphate (NH4+-N) increased the saprophytic growth of T. koningii in sterile soil whereas nitrogen added as nitrate (NO3--N) suppressed growth of T. koningii. Saprophytic growth of T. koningii was markedly reduced in the presence of a natural soil microflora, and enhanced in soil sterilised with ethylene oxide and, to a lesser extent, in soil pasteurised by microwave treatment. The soil sandwich technique could be used as a screen to identify soils and soil factors conducive to Trichoderma establishment and growth.