Microbacterium trichotecenolyticum B4-1, a strain that produces soluble microbial products (SMPs) of protein substances, was employed to investigate SMP production under various substrate levels. This study also estimated the effect of typical operating condition of membrane bioreactor (MBR) under an extremely low substrate level. It was observed that soluble protein production was apparent under a substrate-rich environment, but can be negligible under a normal substrate level. The concentration of soluble protein was directly proportional to initial food to microorganism (F/M) ratio, which the soluble protein can be regarded as utilization-association products (UAPs). Under the extremely low substrate level, soluble protein of possible biomass-associated products (BAPs) was released in correlation with cell decay to 74.0 mg/L, which was higher than that under the normal substrate level. In addition, the BAP protein had the highest potential to cause membrane fouling, increasing the total membrane resistance to 89.3 x 10(11) m(-1) after 20-h filtration. Excitation-emission matrix (EEM) analysis was also applied to determine the composition of the UAPs and the BAPs. The EEM spectrum of humic-acid and fulvic-acid substances between the UAP and the BAP were different. In this study, it was shown that the fouling potential was more closely related to the SMP composition than the SMP concentration. (C) 2011 Elsevier B.V. All rights reserved.