A rheological method for the evaluation of volume fraction (φ) of cell suspensions is discussed with special regard to Saccharomyces cerevisiae. Experimental results can be fitted by a quadratic expression of φ as it is expected when Einstein's formula for a suspension of rigid and spherical particles is referred to. The coefficients however, seem to assume values different from the theoretical ones. As examples of direct application, growth curves and osmotic volume changes are shown to give, thereby, 1.2×10^-1 mol/l as the equivalent osmotic pressure of a yeast cell. The correct value of φ thus obtained might also serve to compute various other cell quantities from observed physico-chemical properties of the whole suspension.