Methods have been developed to study the biological oxidation of plastic fragments in natural soils and sewage sludge using a modification of the conventional Warburg apparatus. Rates of biodegradation are estimated from the consumption of oxygen under carefully controlled conditions. Whereas high-molecular-weight plastic particles are shown to be resistant to biodegradation, degraded branched polyethylene and polypropylene show significant rates of oxidation in both environments. Degraded polystyrene appears to be more resistant to microbial attack.