A heterologous phosphotriesterase (parathion hydrolase) was previosly shown to be secreted by Streptomyces lividans. To investigate the mechanism of secretion, a system to label the protein and follow its expression and secretion was developed. The recombinant S. lividans was grown first in a defined medium containing [35S]methionine that permitted expression but not secretion. It was then transferred to tryptone/glucose medium with unlabeled methionine for the chase period, during which secretion was observed. The results indicate a relatively slow rate of secretion that is also dependent on the growth medium.