Enzyme histochemical techniques were used as markers of macrophage activity and differentiation in the periodontal tissues following orthodontic tooth movement in man. The enzymes studied included lactate dehydrogenase, glucose-6-phosphate dehydrogenase, succinic dehydrogenase, acid phosphatase and its tartrate resistant isoenzyme, arylsulfatase, aminopeptidase M and prostaglandin synthetase. Chloroacetyl esterase activity was studied in order to detect possible neutrophilic degrading activity. Intense activities of arylsulfatase and prostaglandin synthetase and a moderate activity of aminopeptidase M were found in cells degrading the hyaline zone. However, no activity of tartrate resistant acid phosphatase was found in these cells. Giant cells in contact with bone surfaces adjacent to the hyaline zone exhibited an intense activity of succinic dehydrogenase, tartrate resistant acid phosphatase and aminopeptidase M. Chloroacetyl esterase activity did not change following orthodontic treatment. The results indicate that macrophages in various stages of differentiation were responsible for the degradation of the hyaline zone and alveolar bone during orthodontic tooth movement. The enzymatic differences were probably due to the influence of the immediate cellular environment. Prostaglandin synthetase activity, which may be interpreted as a sign of prostaglandin secretion, was associated with the degradation of the hyaline zone in man.