Vacuolation in fibroblasts cultivated in the presence of sucrose is associated with progressive accumulation of the undigestible sugar. In radioisotope experiments the process lasted several days, and when the cells were subcultured back into a medium devoid of sucrose the label was also lost after several days. This type of vacuolated cells is more fragile when it is challenged with lytic agents. 51Cr-labelled LS fibroblasts released more radioactivity when they had been growing in the presence of sucrose, whether they were suspended in media of decreasing osmolarity, in dilutions of various surfactants, exposed to high temperatures, or subjected to mechanical stress. It is concluded that these cells exhibit a lower resistance when exposed to unfavourable environments, but retain their viability in growth media despite some morphological and biochemical alterations.