An extracellular aspartic proteinase (Rmap) from Rhizopus microsporus var. rhizopodiformis was detected in the culture supernatant of a fungal isolate from a case of rhinocerebral mucormycosis (case HA). The proteinase was purified to near homogeneity by ion exchange and affinity chromatography on pepstatin agarose. Based on its N-terminus the RMAP gene was cloned and found to code for 388 amino acids. The preproenzyme has an aminoterminal leader sequence of 65 amino acids, whereas the mature enzyme consists of 323 amino acids. The deduced amino-acid sequence of the preproenzyme was 82% homologous to an extracellular aspartic proteinase of Rhizopus niveus. Low stringency Southern blot analysis of R. microsporus DNA suggested the presence of other homologous genes. Expression of Rmap in Pichia pastoris was achieved, and the recombinant enzyme was active in the yeast culture supernatant. Both enzyme preparations exhibited a similar optimum of activity in the pH 2.5 region. Furthermore, Rmap was shown to activate bovine blood coagulation factor X at slightly acidic pH in vitro. Expression of the proteinase during mycosis was proven by a specific immune response of patient HA.