The secretion of the heterologous parathion phosphotriesterase in S. lividans using the Streptomyces beta-galactosidase signal sequence was further characterised using a pulse/chase system, Unsecreted cell-associated protein in both the precursor and signal-cleaved forms was observed when the protein was expressed from both low- and high-copy vectors, Fractionation of the cells followed by immunoprecipitation with phosphotriesterase antibody suggests that the precursor is membrane-bound while the signal cleaved form is present in the soluble fraction, Preliminary data on the processing of alpha-amylase, a native Streptomyces protein, showed much more rapid processing and secretion, but nevertheless still revealed cell-associated, signal-cleaved protein.