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Summary The formation of callus tissues from root segments of Zea mays seedlings was generally induced on synthetic media by 2, 4-D at concentrations ranging from 2 to 20 mg/1; the subcultures of this callus tissue, however, were successfully cultivated at lower auxin concentrations (1 — 4 mg/1). Studies on the uptake and incorporation of ( 3 H-methyl)-thymidine by root segments grown on a medium containing 5 mg/1 2,4-D have shown that the thymidine incorporation increased after a lag phase of 6 h and then reached two maxima: The first one after 24 h and the second one after 120 h. By treatment with specific inhibitors, actinomycin D (10 −5 and 10 −6 M), puromycin (10 −4 and 10 −5 M) or cycloheximide (10 −4 and 10 −5 M) the thymidine incorporation during the first period can be inhibited either partially or completely. The total nucleic acids prepared by phenol extraction were separated by MAK-column chromatography. In order to demonstrate the incorporation of 3 H-thymidine into DNA the nucleic acids from all fractions of the resulting DNA-RNA-peak were again precipitated and furtherly characterized. The nucleic acids from the DNA-RNA-peak show a very high specific activity and are the only one labelled with 3 H-thymidine. They are stable against RNase but not against pancreatic DNase I. Preparations from callusing roots show a higher 3 H-thymidine incorporation and a higher specific activity in comparison to normal roots. From these findings it was concluded that an additional DNA-synthesis is necessary in the early period of callus formation.