Six octopine tumour lines incited by pTiB6S3, pTiAch5 and pTiA6 on tobacco, Arabidopsis and Petunia were studied by the Southern blotting hybridisation technique in order to define accurately the dimensions of the segments of plasmid origin transferred to the tumourous cell and their organisation in the plant genome. Emphasis has been put on the comparison between octopine and nopaline T-DNAs and on the lines presented here compared with those studied previously (Thomashow et al. 1980). The length of the transferred DNA segment does not depend on the plasmids used, nor on the host plants. The octopine T-DNA organisation in the cell nucleus is significantly different from that of nopaline T-DNAs: tandem arrangements of T-DNA segments could not be detected and the T-DNA itself is much shorter. The tumour lines described here can be compared to some extent with those studied by another group (Thomashow et al. 1980) by the same technique. However, some differences were observed. The transferred DNA was seen as a unique stretch of about 11 kb present only once per cell. No amplification of any part was noticed in any of these six lines. Examination of the restriction patterns presented by the boundary fragments of the T-DNA in these lines suggested that some of them were of common origin.