A procedure is described for measuring Escherichia coli signal peptidase I activity which exploits an intact precursor protein composed of the alkaline phosphatase signal peptide fused to the full length mammalian cytochrome b5. This cytochrome b5 precursor protein has been extensively characterised and shown to be processed accurately by purified signal peptidase I [Protein Expr. Purif. 7 (1996) 237]. The amphipathic, chimaeric cytochrome b5 precursor was isolated in mg quantities in a highly homogeneous state under non-denaturing conditions. The processing of the cytochrome b5 precursor by signal peptidase displayed Michaelis–Menten kinetics with Km=50 μM and kcat=11 s−1. The Km was 20-fold lower than that obtained with signal peptide substrates and 3-fold higher than that reported for pro-OmpA-nuclease A precursor fusion. The corresponding turnover number, kcat, was four orders of magnitude greater than the peptide substrates but was 2-fold lower than pro-OmpA-nuclease A precursor fusion. These results confirm that both the affinities and the catalytic power of the signal peptidase are significantly higher for macromolecular precursor substrates than for the shorter signal peptide substrates.