The molecular weight reduction of low-density polyethylene (LDPE) buried under bioactive soil for 32-37 years was examined by GPC. Microscope IR spectra of the surface of the degraded sample showed the characteristic feature of biodegradation, while the inner part of the sample was almost unchanged. The number average molecular weight (M-n) of the surface was reduced to almost one-half of the inner part. Moreover, a low-molecular-weight component around a molecular weight of 10(3) was recognized for degraded samples. SEM observation suggested that the formation of the low-molecular-weight components is the result of erosion due to enzymatic reaction. The results give evidence for the biodegradation of high-molecular-weight LDPE.