An inhibitor of collagenase was released from cultured cervical tissue of sheep and the amounts released were greatest from tissue in late pregnancy (145-146 days). The molecular weights of material with inhibitory activity, estimated by gel filtration of extracted inhibitor, at different stages of pregnancy were different, i.e. 20,000 from extracts of late-pregnant and post-partum samples and 29,000 from extracts of non-pregnant samples. Inhibitor from culture supernatants had a molecular weight of 42,000. We conclude that the inhibitor has binding characteristics and molecular weights similar to those of the tissue inhibitor of metalloproteinase.