In guinea pig dorsal skin the semicarbazide-sensitive amine oxidase (SSAO) is localised in fibroblasts. Fibroblasts in culture lose the ability to express this enzymatic activity with doublings, thus suggesting that the SSAO expression needs some factors which are not present in the 10% bovine serum culture medium. Fresh bovine serum of adult animals contains two SSAO activities, one with high affinity for benzylamine and one with lower affinity. The enzyme with lower affinity for benzylamine was identified as spermine oxidase, the oxidation of [14C]-benzylamine was inhibited by semicarbazide, alpha-aminoguanidine and B24, a specific inhibitor of benzylamine oxidase and spermine oxidase, both SSAO enzymes. The enzymatic activity of bovine serum was partially purified, the kinetic properties and sensitivity to inhibitors studied. A mathematical procedure for the analysis of the kinetics resulting from the activity of two enzymes acting on the same substrate seems to give better results than the methods previously described.