Six structurally similar and strongly cationic peptides belonging to the brevinin-1 family were isolated from skin secretions of the plains leopard frog Lithobates blairi and the lowland leopard frog Lithobates yavapaiensis on the basis of their antimicrobial activities. Brevinin-1BLc (FLPIIAGIAAKFLPKIFCTISKKC) from L. blairi represented the most potent peptide (MIC=25microM Escherichia coli, MIC=1.5microM Staphylococcus aureus, MIC=3microM Candida albicans, LC(50)=9microM human erythrocytes and LC(50)=6microM HepG2 human hepatoma-derived cells). The appreciably lower antimicrobial potencies of brevinin-1Ya and -1Yc from L. yavapaiensis correlate with the decreases in cationicity produced by the amino acid substitutions Lys(11)-->Asn (brevinin-1Ya) and Pro(14)-->Glu (brevinin-1Yc). In addition, a peptide isolated from the skin secretions of L. yavapaiensis belonging to the ranatuerin-2 family (ranatuerin-2Ya; GLMDTIKGVAKTVAASWLDKLKCKIT GC) inhibited the growth of E. coli (MIC=50microM) and S. aureus (MIC=50microM). In contrast to brevinin-1BLc, ranatuerin-2Ya showed appreciably greater cytolytic activity against HepG2 cells (LC(50)=20microM) than against erythrocytes (LC(50)>100microM).