A directed search for antibiotics active in a syncytium formation inhibition assay using a co-culture of HeLa-T4 cells expressing CD4 antigen and BSC-1 cells expressing gp-120 led to the isolation of pradimicin S, a new member of the pradimicin family. The producing strain (AA0851) was characterized as a new species of Actinomadura for which the name Actinomadura spinosa sp. nov. is proposed. Production of pradimicin S by strain AA0851 was significantly improved by addition of ferrous sulfate (0.1-0.4%) to the production medium. Pradimicin S showed potent activity against human immunodeficiency virus (HIV) LAVBRU strain in a CPE assay, and moderate activity against influenza virus type A. The antibiotic was active against a wide variety of fungi and yeasts in vitro and demonstrated in vivo efficacy against a systemic Candida albicans infection in mice.