Xylocandins A1, A2, B1, B2, C1, C2, D1 and D2 are new antifungal peptides isolated from Pseudomonas cepacia ATCC 39277. The molecular weights of the xylocandins were determined by fast atom bombardment mass spectrometry (A1 m/z 1,215; A2 1,199; B1 1,229; B2 1,213; C1 1,097; C2 1,081; D1 1,083; D2 1,067). Each xylocandin is a cyclic peptide containing glycine, serine, asparagine (1-3 residues), beta-hydroxytyrosine, and an unusual amino acid with the formula C18H37NO5. Additionally A1, A2, D1 and D2 contain 2,4-diaminobutyric acid; A1, B1, C1 and D1 contain erythro-beta-hydroxyasparagine; and A1, A2, B1 and B2 contain xylose. For each xylocandin pair, an erythro-beta-hydroxyasparagine residue in the first component of the pair is thus replaced by an asparagine in the second component, accounting for the 16 dalton mass difference for each pair. Chemical modification of A1 and A2 at the diaminobutyric acid and beta-hydroxytyrosine residues was used to probe structural requirements for activity.