When the Legionnaires' disease (LD) bacterium is grown on supplemented Mueller-Hinton agar, brown pigmentation of the medium occurs. Since this browning may result from tyrosinase-mediated formation of melanin, we supplemented yeast-extract agar with various aromatic precursors of melanin and inoculated it with eight strains of the LD bacterium. Browning occurred with growth of each LD strain of the bacterium on yeast-extract agar enriched with 2.5 mmol/L of L-phenylalanine or L-tyrosine but not without such enrichment. Equimolar D-phenylalanine or D-tyrosine in yeast-extract agar did not enhance browning. The LD bacterium may possess L-phenylalanine hydroxylase activity, but it does not use D-aromatic amino acids effectively in pigment production.