L-Proline entered both mycelial and yeast cells of Candida albicans by an active transport system of high specificity at low (less than 0.1 mM) external concentrations of substrate. The apparent Km value of this system was 0.1 mM for both types of cells, while the V value was 4 nmol min-1 (mg dry wt)-1 for mycelial cells and 1.4 nmol min-1 (mg dry wt)-1 for yeast cells. At L-proline concentrations greater than 0.1 mM, the amino acid appeared to enter both morphological forms by diffusion as well as active transport. As saturation was approached diffusion became increasingly important. The higher uptake rate of mycelial cells seemed not to be the result of an inducible system. The optimal pH and temperature for transport of L-proline were 7.0 and 37 degrees C, respectively. Sodium azide and the proline analogues sarcosine and L-azetidine-2-carboxylic acid inhibited L-proline uptake, while L-thiazolidine-4-carboxylic acid was less effective. The active transport system was highly specific for L-proline since neither ammonium ions, which inhibit the general amino acid transport system of fungi, nor 16 different amino acids interfered substantially with uptake.