Four new dipeptidyl nikkomycins of the Z- and the X-series with a variation in the amino acid moiety of the molecule were isolated from the mutant Streptomyces tendae 901/395 and characterized. Nikkomycins K_z and K_x contain 2-amino-4-hydroxy-4-(2-pyridyl)butyric acid, and nikkomycins O_z and O_x 2-amino-4-hydroxy-4-(5-hydroxy-2-pyridyl)butyric acid. In contrast to nikkomycins Z and X, nikkomycins K_x, and O_x are quite stable at alkaline pH and exhibit a lower biological activity against various test organisms. From the mutant S. tendae 901/C37, which is auxotrophic for methionine and threonine, enhanced amounts of two tripeptidyl nikkomycins, Q_z and Q_x, were produced which are analogues of nikkomycins J and I and contain a homoserine residue instead of glutamic acid. These nikkomycins exhibit a high pH instability.