To distinguish between biotic and abiotic processes in laboratory experiments with environmental samples, an effective sterilization method is required that prevents biological activity but does not change physico-geochemical properties of samples. We compared standard sterilization methods with respect to their impact on microbial abundance and activity. We exposed marine sediment to (i) autoclaving, (ii) gamma-radiation or (iii) sodium azide (NaN.sub.3) and determined how nucleic acids, microbial productivity, colony forming units (CFUs) and community composition of microorganisms, fungi, unicellular protists and protozoa were affected. In autoclaved and gamma-sterilized sediments, only few colonies formed within 16 days. After addition of NaN.sub.3 to the sediment, numerous CFUs (>50) but lower .sup.3H-leucine incorporation rates, i.e. lower protein biosynthesis rates, were found compared to the other two sterilization techniques. Extractable RNA was detected immediately after all sterilization treatments (0.2-17.9 ng/g dry sediment) but decreased substantially by 84%-98% after 16 days of incubation. The total organic carbon content increased from 18 mg L.sup.-1 to 220 mg L.sup.-1 (autoclaving) and 150 mg L.sup.-1 (gamma-radiation) after sterilization. We compare advantages and disadvantages for each tested sterilization method and provide a helpful decision-making resource for choosing the appropriate sterilization technique for environmental studies, particularly for marine sediments. Keywords: autoclaving; gamma-radiation; sodium azide; .sup.3H-leucine; Mossbauer spectroscopy; T-RFLP; RNA