The protein profile of Trichoderma harzianum IMI 275950 growing on two contrasting substrates was compared by gel filtration and SDS polyacrylamide gel electrophoresis. The media selected had either glucose or ground straw as the carbon source. Crude extracts and fractions purified by gel filtration from the two media were used to raise polyclonal antisera in rabbits. The protein profile of the T. harzianum was affected greatly by the growth medium. The reactivity of the antisera to T. harzianum depended on which medium the fungus had been cultured in. This complicates the development of serological assays to quantify T. harzianum biomass in environmental samples. Also, there were varying degrees of reaction to other strains of Trichoderma, Gliocladium virens, and Phomopsis sclerotioides, but none to arrange of other fungi.