The break-up phenomena occurring in a high pressure homogenizer equipped with an interchangeable orifice valve were investigated by measuring the inactivation of Lactococcus laths. Data were collected at varying the orifice size (80, 100, and 150 mu m), the operating pressure (100-200 MPa), the number of passes (1-10), and the fluid viscosity (2.5-7.9 mPa s, changed by adding 0-50 % wt PEG 200 to buffered peptone water) to identify the correlations of the fragmentation occurring in the valve with the main fluid dynamic phenomena (turbulence, elongational and shear stresses, and cavitation). In addition, also the effects of a purely shearing or ultrasound treatment on cell break-up were considered. The results show that the most intense break-up phenomena occur for the smallest orifice size, highest pressure, and lowest viscosity. However, at low viscosity, turbulence, together with the elongational stresses appear to be the controlling factors of cell break-up, whereas, at higher viscosities, the shear stresses become increasingly important. The occurrence of cavitation is only slightly affected by viscosity, and mainly depends on the velocities reached in the homogenization valve.