Callus was induced from tobacco pith and subcultured with frequent transfers on Murashige and Skoog modified medium containing 13.5 mg/l IAA and 0.64 mg/i benzyl adenine. The growth curve by measuring the increase of protein content was a typical sigmoid curve for the callus tissue. The total activities of IAA oxidase and peroxidase were not parallel. IAA oxidase activity was low from 8-26 culture-days and increased after 30 days. The activity of peroxidase decreased slightly at an early stage then increased after 12 days which was earlier than that of IAA oxidase. Peroxisdae was separated into six isozymes (A, B, E, F, M, N) by disc polyacrylamide gel electrophoresis at 18 days. A correlation was apparent between the relative levels of the peroxidase isozymes and the rate of growth. The development of slower-migrating peroxidase isozyme A and B was accompanied by a rapid growth rate at the log phase. The isoperoxidases of fast-migrating rate (E, F, M, N) were associated with growth retardation. The zymograph of IAA oxidase after electrophoresis separation was similar to that of peroxidase during the growth period. Peroxidase A, E, F and N possessed IAA oxidase activity, however, isoperoxidasc B did not parallel with that of IAA oxidase. Whether these two enzymes were at the same apoenzyme or not was established.