The measurement of acyl-CoA dehydrogenase activity is an essential part of the investigation of patients with suspected defects of fatty acid oxidation, and recently the organometallic oxidant ferricenium hexafluorophosphate has been introduced as an electron acceptor for these assays. However, we show that when medium-chain acyl-CoA dehydrogenase activity was measured in cultured skin fibroblasts and platelets from patients with proven defects of this enzyme, there was considerable residual enzyme activity when this electron acceptor was used. The ferricenium assay is not as specific as the anaerobic ETF-linked assay in the biochemical diagnosis of medium-chain acyl-CoA dehydrogenase deficiency in fibroblasts, and therefore is of limited clinical applicability in its present form.