Gadolinium magnetic resonance contrast agents are known to interfere with some clinical chemistry tests, particularly colorimetric assays for serum calcium. We studied the effects of 4 agents, gadodiamide, gadoversetamide, gadopentetate dimeglumine, and gadoteridol, for interference with multiple serum assays. Gadodiamide and gadoversetamide produced clinically significant negative interference with colorimetric assays for serum angiotensin-converting enzyme, calcium, and zinc. These agents produced clinically significant positive interference in magnesium and total iron binding capacity assays and both positive and negative interference in iron assays. Gadopentetate dimeglumine produced a negative interference with iron assays, and gadopentetate dimeglumine and gadoteridol produced negative interference with a colorimetric zinc assay. Caution should be exercised when using colorimetric assays for angiotensin-converting enzyme, calcium, iron, magnesium, total iron binding capacity, and zinc in serum samples from patients who have recently received magnetic resonance contrast agents. In general, gadodiamide and gadoversetamide are more likely to produce a clinically significant interference than gadopentetate dimeglumine and gadoteridol. Likewise, certain analytic methods are more prone to interference, while others not affected.