Currently, the method most used for measuring the maximum specific growth rate (mu(Hmax)) of heterotrophic biomass is by respirometry, using growth batch tests performed at high food/microorganism ratio. No other technique has been suggested, although the former approach was criticized for providing kinetic constants that could be unrepresentative of the original biomass. An alternative method (seed-increments) is proposed, which relies on measuring the initial rates of respiration (r(O2-ini)) at different seeding levels, in a single batch, and in the presence of excess readily biodegradable substrate (S-S). The ASM1-based underlying equations were developed, which showed that mu(Hmax) could be estimated through the slope of the linear function of r(O2-ini)(V-WW + v(ML)) vs v(ML) (volume of mixed liquor inoculum); V-WW represent the wastewater volume added. The procedure was tested, being easy to apply; the postulated linearity was constantly observed and the method is claimed to measure the characteristics of the biomass of interest. (C) 2012 Elsevier Ltd. All rights reserved.