In cultured fibroblasts of adrenoleukodystrophy (ALD) patients and most heterozygotes, concentrations of long-chain fatty acids (greater than C22) were significantly higher than in controls when cells were assayed 4-5 days after reaching confluence. Intermediate values were found in three independent cultures of a girl with manifest ALD, suggesting that a significant proportion of her fibroblasts does not express the defect. Though long-chain fatty acid concentrations in heterozygotes were somewhat higher than expected, suggesting a slight preponderance of defective cells, it may be too early to conclude that somatic selection is a consistent finding in cultured fibroblasts of ALD carriers.