A collection of actinomycetes including fresh isolates was initially screened for the ability to degrade ball-milled straw or utilize lignin-related aromatic compounds. Selected strains were tested for ligninolytic activity by measuring the amount of14CO2 released from [14C-lignin] wheat lignocellulose. Two actinomycetes,Thermomonospom mesophila and aStreptomyces sp., were particularly effective, degrading up to 8% of the radiolabeled substrate to14CO2 in 10 d at 37‡C.14CO2 evolution was not significantly affected by flushing flasks with air rather than 100% O2, or growing the actinomycetes in shake-flask rather than stationary broth cultures. Solubilization of radioactivity paralleled14CO2 evolution and was greatest during the first 72 h of growth, after which no further increase in water-soluble14C was detected although14CO2 evolution continued at a reduced rate. The regulation of ligninolytic activity in these actinomycetes thus differs from that in white-rot fungi, and HPLC analyses of the degradation products suggest that their mode of attack on grass lignin is distinct.