A simplified method using centrifugation and microcentrifuge tubes instead of filtration to measure bacterial activity using thymidine or leucine incorporation, originally used in aquatic habitats, has been adapted for soil. A final step employing hot NaOH to solubilize macromolecules before adding scintillation fluid was necessary to achieve the same incorporation rates as those in the filtration technique. The microcentrifugation technique has several advantages, including being less costly, less laborious and having lower zero-time controls. The samples could also be stored for at least 2 weeks after incorporation was stopped by adding TCA. The microcentrifugation technique would therefore be most useful when a large number of samples, some with very low incorporation rates, are to be studied, for example, when studying bacterial community tolerance. (C) 2001 Elsevier Science Ltd. All rights reserved.