The effect of alpha-amino acids ("tailor-made" additives) on the morphology and nucleation of glycine crystals was studied in batch experiments, and the effect of L-leucine on glycine crystal growth kinetics was investigated in flow cell experiments at constant supersaturation. From dissolution and growth experiments in a flow cell system, L-leucine was found to adsorb onto the (0 (1) over bar 0) face of glycine crystals and to inhibit the growth of that face, modifying the crystal habit. At high L-leucine concentrations (>8 mg/ml), the growth rate of the (010), (011), and (01(1) over bar) faces was faster than that in the absence of additive. This may be the result of the effect L-leucine has on the crystal/solution interfacial tension and on the aggregation of glycine in solution. Oriented nucleation and growth of glycine crystals on the (0(1) over bar0$) face were observed. This may be a consequence of multilayer L-leucine adsorption onto the (0(1) over bar0$) face, which provides a template for nucleation and growth of glycine twinned crystals about the (0(1) over bar0$) basal plane.