Changes in fermentation media not only affect the performance of the fermentation itself (with regard to the kinetics of biomass and product formation and the yields obtained) but also the initial product-recovery operations downstream of the fermenter. In this work, microfiltration experiments to remove Saccharopolyspora erythraea biomass from fermentation broth and to recover erythromycin were carried out using two fundamentally different media; a soluble complex medium (SCM) and an oil-based process medium (OBM). Small-scale batch fermentations of 14-L working volume were carried out in triplicate using both media. Broth samples were taken from each fermentation at regular intervals from the end of the exponential-growth phase onwards. These were then processed using a Minitan II (acrylic), tangential crossflow-filtration module, fitted with a single 60 cm(2) Durapore hydrophilic 0.2 mu m membrane, operated in concentration mode. The OEM fermentations produced higher titers of erythromycin but required longer fermentation times due to increased lag phases and slower maximum-growth rates. The OEM also increased the loading on the membrane; at maximum product titers residual oil concentrations of 3 g . L-1, antifoam concentrations of 2 g . L-1 and flour concentrations estimated at approximately 10 g/L-1 were typical. It was found that both the permeate flux a nd erythromycin transmission were affected by the choice of medium. The OEM had significantly lower values for both parameters (12.8 Lm(-2) h(-1) and 89.6% respectively) than the SCM (35.9 Lm(-2) h(-1) and 96.7% respectively) when the fermentations were harvested at maximum erythromycin titers. Transmission of erythromycin stayed approximately constant as a function of fermentation time for both media, however, for the OEM the permeate flux decreased with time which correlated with an increase in broth viscosity. The relatively poor microfiltration performance of the OEM medium was, however, offset by the higher titers of erythromycin that were achieved during the fermentation. The filtration characteristics of the SCM broth did not show any correlation with either broth viscosity or fermentation time. image-analysis data suggested that there was a correlation between hyphal morphology (main hyphal length) and permeate flux (no such correlation was found for the OEM broth). Moreover, it has been shown for the OEM broth that the residual flour had a profound effect on the microfiltration characteristics. The influence of the residual flour was greater than that imposed by the morphology and concentration of the biomass. The understanding of the factors governing the interaction of the fermentation and microfiltration operations obtained in this work provides a first step towards optimization of the overall process sequence.