We describe the isolation by reversed-phase h.p.l.c. of a number of products of the degradation of insulin by insulin proteinase and their direct analysis by fast atom bombardment mass spectrometry (f.a.b.-m.s.). Various semisynthetically labelled insulins were used, including [[2H2]GlyA1]insulin and [18O]LysB29]insulin. The results obtained confirm and extend the results obtained by non-mass-spectrometric methods [Davies, Muir, Rose & Offord (1988) Biochem. J. 249, 209-214, and papers cited therein]. Cleavage sites were identified between positions A13-A14, A14-A15, B9-B10, B13-B14, B24-B25 and B25-B26. The advantages and disadvantages of the application of f.a.b.-m.s. to such studies are discussed.