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Crystalline cellulose was anaerobically degraded using a leachate inoculum derived from simulated municipal solid waste. Bicinchoninic Acid (BCA) protein assays were used to measure the distribution of biomass during cellulose degradation, including the planktonic and sessile biomass fractions. A comparison of sessile and planktonic microbial growth indicated that the microbial growth was dominated by the planktonic fraction with the biofilms accounting for approximately 25% of the population. Additional biomass measurements were conducted to test the reliability of the BCA protein assays. Total microbial growth was inferred from the accumulation and depletion of ammonia nitrogen measured using flow injection analysis. The planktonic biomass was estimated from direct cell counts using light microscopy and the sessile biomass was estimated by analysing the nitrogen content of the separated and washed cellulose pellet. Regression analysis showed good correlations between the measurement pairs representing the total biomass (R²= 0.90), planktonic biomass (R² = 0.97) and sessile biomass (R² = 0.85), supporting the use of protein assays as an indicator of microbial growth in mixed culture environments. [ABSTRACT FROM AUTHOR]