Sequences of DNA that hybridize on Southern blots with cloned EcoR1 1.3 kb (ER₁) of long interspersed repeated sequence (L1Md) of mouse have been examined in genomic DNA of neonatal mice, livers and brains of adult mice (3, 10, 27, and 30 mo old), and the solid myeloma tumor MOPC‐315. The isoschizomers Hpa II (CCGG or ^mCCGG) and Msp I (CCGG or C^mCGG) were used to assess methylation. We found that the L1Md sequence is fully methylated in young animals but demethylated in myeloma. Demethylation of L1Md sequence also occurred in aged animals. By scanning the autoradiogram, we found that approximately 8% of the 10⁴–10⁵ copies have been demethylated in 27‐mo‐old liver.