AbstractThe ability of a range of actinomycetes to decolorise the polymeric dye Poly R was investigated. Three well characterised lignocellulose‐degrading actinomycetes,Streptomyces viridosporus, Streptomyces badiusandThermomonospora mesophilawere found to be the most efficient at decolorising Poly R, with a maximum decolorisation rate of 0.10 unit per day. Extracellular fractions taken fromS. viridosporusgrown on a variety lignocellulose‐related substrates were also capable of decolorising Poly R, indicating that dye decolorisation was not merely due to biomass uptake. The activity of extracellular fractions from straw‐grown cultures ofS. viridosporuswas three to six times greater than those from other substrates examined. Purification of this dye‐decolorising activity fromS. viridosporususing anion exchange chromatography revealed the presence of a single active fraction. This was confirmed using gel permeation chromatography which estimated the molecular mass of the decolorising protein to be approximately 3