Crystals of a tetrapeptide derivative N<2>-(N,N-dimethyl-L-valyl)-N-[(1S,2R)-2-methpxy-4-[(2S)-2-[(1R,2R)-1-methoxy-2-methyl-3-oxo-[(2-phenylethyl)amino]propyl]-1-pyrolidinyl]-1-[(S)-1-methylpropyl]-4-oxobutyl]-N-methyl-L-valinamide having a potent antitumor effect or its salt and a process for producing the same.
