Measurements of volume distributions and dry weight are made on hybridoma cells in culture. The volume of viable hybridoma cells is significantly larger than that of nonviable cells. During exponential growth, the volume of the viable hybridoma cells is found to be significantly larger than that during other stages of batch culture. Proportionality is found between the volume of the cells and their dry weight, indicating that the volume data can be used in conjunction with cell concentration data as a practical technique for indirect measurement of the biomass concentration present in a culture. Comparison of dry weight concentrations in continuous culture to predictions from the volume data shows very good agreement.