Updated project metadata. We improve on currently-available resources by describing a mass spectrometry (MS)-based strategy using stable isotope dynamic labelling of secretomes (SIDLS) that discriminates between authentic secretory proteins and intracellular proteins within the secretome of cultured cells. By monitoring the rate of incorporation of labelled amino acids into newly synthesised proteins as they appear in the media, we can differentiate those proteins that have been destined for secretion, and exhibit rapid labelling, from those with low rates of labelling or low turnover relative to the growth rate of the cells which is a feature of intracellular proteins.