An unstructured mathematical model, termed the algebraic model, has been developed for predicting cellular activity of a liquid phase batch culture. This rate model is based on the natural order of cellular processes, namely substrate uptake by the cell, followed by conversion into energy and new cells. The specific substrate uptake rate is modelled by a hyperbolic Michaelis-Menten-type equation, as a function of the environmental substrate concentration. Conversion of carbon substrate into new cells and energy is modelled by a variable yield concept, which incorporates the requirements of cell internal maintenance. The algebraic model has been used to analyse experimental batch-culture darn with a large range of initial substrate and cell concentrations. The kinetic parameter values obtained from this analysis were used to simulate complete batch-culture fermentations. The comparison of the experimental biomass and substrate time-course data with the simulations given by the algebraic model, showed that the simulation was highly accurate. Hence, the algebraic model is a new and useful means of simulating and assessing microbial cell activity and has the potential for further development through the addition of modular functions.