Onconase and bovine seminal RNase, two members of the RNase A superfamily, inhibit human immunodeficiency virus type 1 replication in H9 leukemia cells 90-99.9% over a 4-day incubation at concentrations not toxic to uninfected H9 cells. Two other members of the same protein family, bovine pancreatic RNase A and human eosinophil-derived neurotoxin, have no detectable antiviral activity, demonstrating a strikingly selective antiviral activity among homologous ribonucleases. The antiviral RNases do not appear to affect viral particles directly but inhibit replication in host cell cultures. Onconase, already in clinical trials for cancer therapy, and bovine seminal RNase have potential as antiviral therapeutics.