Biotechnology Techniques Vol 5 No 1 39-42 (1991) Received as revised 18th December A SIMPLE AND RELIABLE METHOD FOR THE DETERMINATION OF CELLULAR RNA CONTENT. S. Benthin*, J. Nielsen and J. Villadsen Department of Biotechnology, Building 223, DK-2800 Lyngby. Technical University of Denmark. SUMMARY. This communication describes a rapid, simple and reliable method for the determina- tion of the cellular RNA content. In the study of microbial growth and product for- mation the cellular RNA content is a good measure of the activity of the biomass. Results from applying the method to monitor transient experiments in a chemostat with Lactococcus cremoris are presented. INTRODUCTION. The intracellular RNA content of microorganisms is well correlated with the size of the protein synthesizing machinery (Maalce, 1972), and the cellular RNA content is, therefore, an important variable in the study of microbial physiology. In simple struc- tured models for microbial growth and product formation the cellular RNA content has succesfully been used to describe the state of the biomass (Esener et al., 1982; Nielsen et al., 1990). For many different microorganisms the cellular RNA content increases monotonically with the specific growth rate for different growth limiting substrates (sugars, amino acids, etc, Maal0e, 1972). The intracellular RNA content is also an interesting variable during hybridoma cultivations since the antibody production only occurs when the RNA content is high (Dalili and Ollis, 1990). Irrespective of the microorganism, measurement of the cellular RNA content during transients in a che- mostat gives fundamental insight into the growth dynamics. As seen in the literature from the last decade, researchers often resort to the Schneider method of hot perchloric acid hydrolysis of RNA and DNA (J6bses et al., 1985, Nielsen et al., 1990). Because both RNA and DNA are hydrolysed the orcinol method is used to determine RNA. The orcinol method is quite laborious and relies on a spectropho- tometric determination of a derivative of ribose. In this communication we propose a reappraisal of the simple UV-spectrophotometric determination of RNA specifically degraded by alkali and extracted in perchloric acid (Munro and Fleck, 1962). 39