Overexpression of the target gene in a recombinant strain of Escherichia coli has been analyzed in view of changes in the carbon-nitrogen : source and as a function of C/N ratio. The rate of target gene expression varied over a range of 200%, and the yield coefficient for the target gene product changed over 400% with change in carbon source at 10 g L(-1) in M9 medium;. With variation in nitrogen source, however, only marginal changes (10-15%) occurred in these parameters of overexpression. Higher concentration, for example 2.5 g L(-1), of any nitrogen source adversely affected heterologous expression as rate of target gene expression declined in the range 35%-50% and the yield coefficient of foreign protein-decreased between 45% and 60%. A C/N ratio of 15 was found to be optimum for both parameters for-overexpression and host specific parameters such as specific growth rate and observed yield coefficient for cellular growth. An analysis with respect to temperature and pH revealed that host and expression parameters were quite susceptible to changes in these factors.