Impurities have a clear negative influence on the nucleation and growth rate kinetics of the semisynthetic antibiotic ampicillin (AMPI) crystallization. The tested impurities phenylglycine and 6-aminopenicillanic acid are the building blocks of AMPI and will therefore be present in any AMPI manufacturing process, as well as the tested AMPI degradation products. The negative impact increases with an increase in the impurity concentration. This is shown by an increase in the induction time of crystallization measured and reported in this paper. The growth rate G (m s(-1)) is related to the supersaturation ratio S of AMPI. The growth rate is determined with a previously developed adapted single-crystal growth rate analysis method (Ottens, M.; et al. Ind. Eng. Chem. Res. 2001, 40, 4821-4827). The mechanism for retardation of the crystallization process is argued to be the blocking of the crystal surface by the impurities.