We have partially purified a soluble protease from Pisum sativum chloroplasts involved in the processing of precursor polypeptides imported into the organelle. The enzyme processes precursors of both stromal and thylakoid proteins to the mature size, but is inactive against all proteins so far tested other than precursors destined for the chloroplast. The enzyme processes precursors from wheat and barley, and is therefore not species-specific. It has a relative molecular mass of about 180 000 and a pH optimum near 9. The enzyme is inhibited by ethylenediamine tetraacetate and 1,10-phenanthroline but not by serine- or thiol-protease inhibitors.