In a systematic search for microbial products active in the syncytia inhibition assay (SIA)1}, two streptomycetes strains AA6532 (ATCC 55290) and AA3891 (ATCC 55286) were found to produce new antiviral antibiotics, named siamycins I and II, respectively. They are peptide group antibiotics structurally related to each other, and both show good inhibitory activity against HIV and HSV viruses. This paper describes the fermentation, isolation, physico-chemical properties and biological activities of siamycins I and II.