Measurements of C3H10T1/2 and V79 cell thickness were performed on living cells by confocal laser fluorescence microscopy. Thickness distributions are reported for cells growing as a monolayer (on mylar and glass) and suspended in their medium. Mean values for cells grown on mylar (corrected for refractive index effects) are 2.9 +/- 0.6 and 6.1 +/- 1.0 microm for C3H10T1/2 and V79 cells respectively. Mean values of the diameters of cells suspended in their medium are 13.0 +/- 1.6 and 9.3 +/- 1.4 microm for C3H10T1/2 and V79 respectively. Knowledge of cell thickness, as irradiated, is of central relevance for studying the relative biological effectiveness of low energy, poorly penetrating radiations. It can be concluded, from the measured cell thickness distributions, that with C3H10T1/2 cells grown on mylar, the LET variation through the whole cell is within 20% for protons and alpha-particles with energies down to 0.6 and 2.5 MeV respectively. From a comparison with thickness values reported in the literature for living or fixed embedded cells growing on plastic substrate, mean values between 2.4 and 3.4 microm and between 6 and 7.5 microm could be assumed for C3H10T1/2 cells and for the most widely used V79 cell lines respectively.