Activated protein C (APC) resistance is related to a single point mutation in the factor V gene (FV:Q506) and appears to be the most common inherited risk factor for venous thromboembolism. A reliable screening test is therefore useful. We aimed to evaluate a new APC resistance test, on the basis of the procoagulant activity present in one snake venom of a crotalidae family: STA Staclot APC-R. We studied 36 consecutive patients with an acute deep venous thrombosis (DVT) confirmed by compression ultrasonography and carrying the FV:Q506allele, assessed by DNA analysis, 103 of their family members and 35 consecutive patients with a proven DVT but who did not carry the FV:Q506allele. Blood samples were collected within 24 h of admission for the DVT cases and on the day of medical registration for the family members. Tests were performed blind. The STA Staclot APC-R test, using a cut-off value of 0.80, had an overall sensitivity of 100% (95% CI, 95–100) and a specificity of 98.8% (95% CI, 92.0–99.6). An acute thrombosis process did not influence the performance of the test. We conclude that this test is easy and rapid to perform in every day practice and fulfills the criteria for a screening test. Blood Coag Fibrinol 9:355–359 × 1998 Lippincott-Raven Publishers.