In our screening program for new antifungal compounds produced by microorganisms, a new natural tetraene macrolide AB-400 has been isolated from culture broth of Streptomyces costae. In addition, this strain produces the known tetraene antibiotic pimaricin. Both antibiotics were found to have very similar structure and were compared by physico-chemical, spectral and microbiological methods. The compound supressed the growth of fungi and the activity is compared with another polyene macrolide, amphotericin B, nystatin, candicidin and pimaricin. The structure of AB-400 and its N-acetyl derivative (Fig. 1) were determined by comparative physico-chemical and spectral analysis with pimaricin and N-acetylpimaricin.