This study was designed to monitor changes in the levels of adenosine 5'-triphosphate (ATP) and deoxyribonucleic acid (DNA) per unit of microbial mass during the autotrophic biodegradation of thiocyanate (SCN-). An artificial medium containing trace minerals and 500 mg SCN-/L was used as a substrate for bacterial growth. An SCN--degrading bioreactor with a working volume of 6 L, equipped with temperature, pH, and dissolved oxygen controls, was operated in batch mode. During the exponential phase of SCN- biodegradation, the ratios of ATP and DNA to microbial dry weight varied from 0.6 to 1.1 mu g ATP/mg of volatile suspended solid (VSS), and from 3.5 to 8.8 mu g DNA/mg of VSS, respectively. The ATP and DNA concentrations correlated linearly with microbial mass (r(2) > 0.9) within the exponential phase. The linear regression equations were as follows: (1) microbial mass concentration (mg/L) = 0.663 x ATP concentration (mu g/L) + 11.1 and (2) microbial concentration (mg/L) = 0.081 x DNA concentration (mu g/L) + 10.9. The applicable ranges were 6.8 to 47.4 mu g/L for ATP concentration and 41.5 to 395 mu g/L for DNA concentration, respectively.