A laboratory-scale membrane filtration cell was constructed to enable direct microscopic observation of microbial cell deposition and release in cross-flow microfiltration. Initial deposition rates determined from microscopic images were interpreted through an interaction force model. Experimentally derived deposition rates and model calculations confirmed that initial cell and particle deposition was governed by permeation drag and electrostatic double layer forces. Microbial deposition rates increased linearly with increasing permeation velocity over several orders of magnitude, but decreased dramatically with small increases to membrane and particle zeta potential. Cross-flow velocity had no effect on initial deposition up to cross-flow Reynolds numbers of 600. Cell deposition rates were significantly lower at high ionic strength and low pH due to cell aggregation in the bulk. Model equations suggested that cell aggregates were more strongly influenced by tangential shear and lift forces, which tend to reduce deposition. The interaction force model predicted at a set of "critical operating conditions" at which cells were reversibly deposited. Direct observation experiments verified that most cells deposited under the predicted "critical" conditions were easily removed when permeation ceased - without the need for back-pulsing or chemical cleaning. Beyond the fundamental knowledge gained from this study, the experimental and theoretical techniques presented may prove valuable in identifying particle and biological fouling potentials of new membrane materials, as well as in developing effective fouling control strategies for environmental membrane separations. (C) 2004 Elsevier B.V. All rights reserved.