Abstract: The invention relates to novel protease-inhibitors which are obtainable from leeches. It also relates to uses thereof, for instance as a medicament, thus pharmaceutical preparations are provided, as are derivatives, mutants, genes encoding, vectors comprising and cells provided with such genes and/or vectors. In particular the invention relates to a family of proteinaceous protease-inhibitors having a molecular weight of about 5.5 kD and the following primary sequences: DDNCGGKVCSKGQLCHDGHCECTPIRCLIFCPNGFAVDENGCELPCSCKHQ, DDDCGGQVCSKGQLCVDGQCKCTPIRCRIYCPKGFEVDENGCELPCTCLQ and DGNCGGQVCSKGQLCVDGQCKCTPIRCRIYCPKGFEVDENGCELPCTCLQ. This invention also relates to HIV-inhibitors and other therapeutically interesting, low molecular weight, and low antigenic substances from leeches.