In this investigation it has been found that naturally-occurring (i.e. indigenous, not transplanted) tumours of diverse organs in a spectrum of vertebrates from frogs to man can secrete enzymes which degrade basement membrane collagens (type IV and V). The enzymes are inhibited by chelating agents (EDTA) but not by other protease antagonists and are, therefore, specific metalloproteases. Individual tumours do not necessarily secrete collagenases active against all collagen types (I, IV and V) and release of these different enzymes does not, therefore, appear to be coordinated. These biochemical findings support those reported for serially transplanted tumour cell lines and provide a plausible mechanism for the destruction of basement membranes and stromal collagen fibres observed morphologically in tumour spread.