Homozygosity for a novel D180G mutation in the protease domain of protein C, associated with plasma protein C activity and antigen levels of 8% of normal was identified in a thrombosis prone family. Transient expression of protein C in HK-293 cells and analysis of protein C antigen in culture media and cell lysates showed that the secretion of mutant protein as compared with wild-type protein was reduced by 79% while the intracellular contents were similar. Computer analysis of the X-ray structure of activated protein C and of a theoretical model of the zymogen predicts that the mutation destabilises the molecule locally. Our results are compatible with a relatively unstable mutant molecule that could be trapped inside the cell and degraded. However, if secreted the mutant molecule could have a relatively normal catalytic activity and structure consistent with the plasma levels of protein C activity and the late onset of thrombosis.