The rates of CO2 evolution induced in five different soils by addition of the d- and l-isomers of the amino acids alanine, glutamine and glutamic acid were measured. The d-amino acid substrateinduced respiration (SIR) rates were all less than the corresponding l-amino acid SIRs, although this difference was not significant for the alanine isomers in every soil. The d- and l-glutamine induced ammonification rate, measured in one of the soils, was also less for the d-isomer than the l-isomer. The difference between the d- and l-amino acid SIRs was greater for glutamine and glutamic acid than for alanine. There were significant relationships between all three l-amino acid SIRs and the glucose SIR, which indicated that l-amino acid SIRs may be used for estimating soil microbial biomass. The d-amino acid SIRs were not constant proportions of the l-amino acid SIRs across all five soils, and there was no significant correlation between either d-glutamine SIR or d-glutamic acid SIR and glucose SIR. The d-glutamine and d-glutamic acid SIRs apparently varied between soils independently of the total microbial biomass but the effects of streptomycin and cycloheximide on amino acid SIRs were inconclusive in establishing whether d-amino acid SIR could be attributed to a particular component of the soil microbial community. Conditioning of one of the soils with peptone, to increase microbial activity, led to an increase in both d- and l-amino acid SIRs. Both the d- and l-alanine SIRs, and the d- and l-glutamic acid SIRs increased by similar proportions following conditioning with peptone, but conditioning with peptone had different effects on d- and l-glutamine SIRs; d-glutamine SIR increased by a factor of 5.6 compared to 3.8 for l-glutamine SIR.