Normal, habituated and transformed in vitro tissue lines of sugar beet (Beta vulgaris L.), horseradish (Armoracia lapathifolia Gilib.) and potato (Solanum tuberosum L.) were studied with regard to isoesterase patterns. Isoenzymes were separated in gradient gels (5-12%) of polyacrylamide and by isoelectric focussing in pH range 4-9. 1- and 2-naphtylacetate were used as substrates of broad spectrum which cover also esterases (arylesterases and carboxylesterases) reacting with organophosphorous compounds. Distinct isoesterase patterns were noticed in sugar beet normal, habituated and crown gall tumour tissues. Horseradish tumour and teratoma, on the contrary, differed only in one anodic isoenzyme. Even the malformed shoots and unorganised tissue of teratoma had the same patterns. In potato tuber tissue, change in isoesterase pattern, characterised by disappearance of a dominant dark area, was observed during tumour development. The gradient gels gave more stable and reproducible isoenzyme patterns than isoelectric focussing.