A model is presented for the disruption of microbial cells in a high-pressure homogeniser. It assumes that cell deformation and breakage due to elongational stresses occur in the high-stress zones close to the surfaces of the valve rod and the impact ring of the homogeniser. The equation based on the proposed model predicts an inverse cube-root dependency of the maximum diameter of cells that remain unbroken upon the pressure drop across the valve, and suggests that the maximum whole cell diameter will increase with an increase in the mechanical integrity of the cell walls. Experimental data are provided for the disruption of batch and continuously grown cells of Saccharomyces cerevisiae. The results from these experiments support, and are interpreted in terms of the predictions from, the proposed model.