The invention provides crystals of N2-(N,N-dimethyl-L-valyl)-N-[(1S,2R)-2-methoxy-4-[(2S)-2-[(1R,2R)-1-methoxy-2-methyl-3-oxo-3-[(2-phenylethyl)amino]propyl]-1-pyrrolidinyl]-1-[(S)-1-methylpropyl]-4-oxobutyl]-N-methyl-L-valinamide
or salts thereof which possess potent antitumor activity and methods for their preparation.