The comparative rates of microbial degradation of 14C-lignin-labeled lignocelluloses and 14C-Kraft lignins were investigated using selected soil and water samples as sources of microorganisms. Natural lignocelluloses containing 14C primarily in their lignin components were prepared by feeding plants uniformly labeled L-[14C]phenylalanine through their cut stems. 14C-Kraft lignins were prepared by pulping lignin-labeled lignocelluloses. Rates of lignin biodegradation were determined by monitoring 14CO2 evolution from incubation mixtures over incubation periods of up to 1000 h. Observed rates of lignin degradation were slow in all cases. Kraft lignins appeared more resistant to microbial attack than natural lignins, even though they were decomposed more rapidly during the first 100-200 h of incubation. Similar degradation patterns were observed in both soil and water. Individual samples, however, varied greatly in their overall rates of degradation of either lignin type. A Kraft-lignin preparation was separated into a variety of molecular weight fractions by column chromatography on LH-20 Sephadex and the biodegradability of the different molecular weight fractions determined. The lower molecular weight fractions of the Kraft lignin were decomposed at a significantly faster rate by the microflora of soil than were the fractions of higher molecular weight.