Biological treatability of an integrated dairy plant wastewater containing a small fraction of whey washwaters mixture has been experimentally investigated. Emphasis has been placed on the assessment of the initially inert fraction, S(I) and soluble residual microbial products, S(P). Parallel batch experiments have been conducted to determine the kinetic and stoichiometric coefficients of the degradable COD. The results have shown that the wastewater tested had practically no initially inert fraction, but generated residual microbial products amounting to 6-7% of the initial degradable COD. The results obtained have been fed into a set of equations describing the steady state operation of an activated sludge system with sludge recycle and a relationship indicating the variation of the total effluent COD with the sludge age has been defined for the wastewater tested. It is noted that effluent COD cannot be biologically reduced below 85 mg l-1, regardless of the sludge age, due to generation of residual fractions.