The growth of immobilized Escherichia coli was analyzed by pulse-chase radioisotope labeling of the cell mass with (35)SO(4) (2-) and subsequent liquid emulsion autoradiography of thin cross sections of the cell aggregate. Bacteria were retained in a planar aggregate on a microporous membrane and grown anaerobically on a phosphate-buffered medium with glucose as the sole carbon and energy source. A mathematical model of immobilized cell growth and convection was used to predict the distribution of label in the cell mass and permit information about both the magnitude and variation in the intrinsic growth rate to be extracted. Growing zone dimensions ranging from 4 to 48 mum and growth rates from 0.28 to 0.5 h(-1) were found. Data collected at low glucose concentrations were consistent with a zero-order description of intrinsic growth kinetics. At high glucose concentrations, conditions under which the system was subject to significant pH inhibition, the data were best described by the prediction of a first-order kinetic model. When coupled with a suitable analytical framework, the combination of radioisotope labeling and autoradiography provides a general method for characterizing immobilized cell growth rates.