A micromanipulation method has been developed to measure the force required to burst single tomato cells (Lycopersicon esculentum vf36) taken from suspension cultures. The method works by compressing a cell between parallel surfaces whilst measuring the force being imposed on the cell, and its deformation. The mean bursting force for two-week-old cells was 3.6 mN (standard error 0.1 mN), at a compression speed of 23 mum s(-1). Usually force-deformation curves showed a single bursting event, but sometimes multiple bursts were observed, implying cells could reseal after failure. If cells were deformed without bursting, and then held, they showed a relaxation of the force. This was attributed to water loss, although wall relaxation was also a possibility. The half time of this relaxation was between 1-10 s. Tests on protoplasts gave bursting forces 1000 fold lower than intact cells, and cells treated with Triton to disrupt the membranes and destroy turgor collapsed with no bursting. As expected, both turgor and the presence of a wall were essential to maintaining cell strength.