Simple techniques are reported for (i) measuring the cake mass formed during crossflow filtration of microbial cells, (ii) recovering and measuring the cake mass after crossflow filtration, and (iii) measuring the specific cake resistance during crossflow filtration. The first technique is based on measuring the decline in cell concentration in the feed reservoir during total recycle operation, Cell concentrations are measured on a dry basis and converted to wet weight concentrations using the previously measured wet-to-dry weight ratio of the cells. The second technique involves a series of flushing steps at zero transmembrane pressure followed by wet weight analysis of the suspension formed by the recovered cake. In the third technique, two types of specific cake resistance are defined; an apparent value, alpha(app), and the true value, alpha(true). These are related by the expression alpha(app) = alpha(true) + (R-m - R-m0)/m where R-m - R-m0 is the flow resistance due to membrane fouling (as distinct from cake formation), and m is the cake mass per unit area. A methodology for decoupling the effects of cake formation and membrane fouling during crossflow filtration is presented, thus, providing a way of monitoring the time evolution of alpha(true). The techniques described were used to investigate the crossflow filtration of the polymorphic microorganism Kluyveromyces marxianus var. marxianus NRRLy2415 in tubular ceramic membranes. This microorganism displays a range of morphologies ranging from simple ovoid yeast to branched filamentous forms. The approach of measuring the cake and membrane resistances, as well as the specific cake resistance, provides a wealth of information, which greatly adds to our understanding of the mechanism of flux decline in complex biological suspensions. (C) 2002 Elsevier Science B.V. All rights reserved.