Modification of the crystallization kinetics of amorphous systems is an important topic, in particular in the area of drug delivery. Herein, we studied the impact of polyhydroxybutyrate (PHB), a biocompatible polymer with a low glass transition temperature (T_g), on crystal growth rates from the supercooled liquid phase of a variety of organic compounds. Low levels of PHB were mixed with the supercooled liquid and growth rates were monitored using optical microscopy at several temperatures above the T_g of the mixture. Growth rates were determined from a plot of crystal size as a function of time. It was found that PHB inhibited the growth of low T_g compounds, while accelerated crystal growth was observed for compounds that had a much higher T_g (>50 °C) than the polymer. The observations were rationalized based on the extent of mobility differences between the crystallizing compound and the polymer. This is the first reported example of a polymer having compound specific inhibitory or accelerator effects on crystal growth.