The cement lines between reparative cementum and resorbed dentin or cementum in human teeth were observed by light microscopy and scanning electron microscopy combined with NaOH maceration. The NaOH maceration was used to remove interfibrillar substances and to observe the fibrillar architecture of the cement lines directly. Light microscopy showed that the cement lines were rich in proteoglycans with mucopolysaccharides, but deficient in collagen fibrils. The cement lines were artificially broken after treatment with hyaluronidase, which digests some of the mucopolysaccharides, but digests no collagen fibrils. Scanning electron microscopy showed that fibril intermingling occurred only in some places between reparative cementum and resorbed tissue. These findings suggested that the proteoglycans in cement lines mediate the attachment between new and old mineralized tissue.