The rates of CO2 production from soils over 6 h in response to addition of either the L- or the D-isomers of alanine, glutamine and glutamic acid have been determined for soils from 13 experimental grassland plots. These plots were established in the 1896-97 winter and have received regular additions of farm yard manure (FYM) either with or without inorganic N, P and K applied, or factorial combinations of inorganic N (as (NH4)2SO4), inorganic P (as basic slag and triple superphosphate) and K (as KCl), or no nutrient addition (control). As a result of these treatments, the soils have contrasting chemical and biological properties. Biomass C, determined from glucose-induced respiration, was well correlated with the L-amino acid- and D-alanine-induced respiration rates. The rates of D-amine acid-induced respiration were consistently lower than those of the corresponding L-isomers, except for alanine in the more acidic soils which had small microbial biomass contents. The D-amino acid-induced respiration accounted for smaller fractions of the variation in microbial biomass than that induced by the corresponding L-isomers. Relative to the control soil, L-amino acid-induced respiration rates were increased by FYM treatment, decreased slightly by the NPK treatment and decreased to a larger extent by the acidifying (NH4)2SO4 treatments. In contrast, the rates of D-glutamine- and D-glutamic acid-induced respiration were reduced by addition of FYM, NPK and (NH4)2SO4. The ratio of L-amino acid-induced respiration to D-amino acid-induced respiration (L-to-D ratio) ranged between 6.3 and 1.4 for glutamine and 8.7 and 2.6 for glutamic acid. The L-to-D ratios for glutamine and glutamic acid increased with increasing soil microbial biomass and declined abruptly, but not linearly, with increasing rate of CO2 evolution per unit of biomass (qCO2). The declines in L-to-D ratios may have been related to increased physiological stress in the soil microbial community, since they occurred along a gradient of increasing soil acidity.