Sucrose presence and concentration modulated in different ways and to different extents the activity of six plant glycoside hydrolases [PGHs] extracted from apple callus cultures, both in the water soluble fraction [WS-F] and in the NaCl-released fraction [NaCl-F]. b-D-Glucosidase activity increased because of sucrose starvation and the addition of sucrose decreased both WS-F and NaCl-Fb-D-glucosidase from calli grown in a Murashige and Skoog's basal medium with [MSH] or without [MS0] plant growth regulators [PGRs]. WS-F and NaCl-F a-L-arabinofuranosidase, NaCl-F b-D-galactosidase and NaCl-F b-Dxylosidase activity reached a maximum when 0.045 M sucrose was added to the MS0 medium with an ensuing decline at higher sucrose concentrations. a-D-Galactosidase and a-D-xylosidase activity reached a maximum when 0.045 M sucrose was supplied and did not decline significantly in 0.09 M sucrose-supplied calli. When the effects of PGR presence or absence were analysed, NaCl-F b-Dglucosidase, a-D-galactosidase, b-D-galactosidase, a-D-xylosidase and b-D-xylosidase activities were found to be higher in MS0 than in MSH. To assess whether sugar effects were sucrose-specific, other sugars [glucose, fructose, galactose, maltose, lactose, raffinose, sorbitol and mannitol] were tested, with or without PGR supplementation. In general, sugar alcohols [mannitol, sorbitol] and some monosaccharides [fructose and glucose in particular] were better inducers of NaCl-F a-L-arabinofuranosidase, b-D-galactosidase and b-D-xylosidase activity than disaccharides [sucrose, maltose, and lactose] or the trisaccharide raffinose. This trend was not widespread to all PGHs assessed since sucrose-supplemented calli displayed higher NaCl-F a-D-galactosidase than those supplemented with glucose, galactose, sorbitol or mannitol. These results show that sugars supplied to callus tissue cultures as a carbon source can also modulate PGH activity. Modulation is different for each PGH, sugar-specific and, at least in the case of sucrose, concentration-dependent. Results also suggest the existence of regulatory interactions between PGRs and sugars as part of an intricate sensing and signalling network. Combination of PGR, sugar type and concentration should be taken into account to maximize each PGH activity for further enzyme studies.