The enzymatic assay of triglyceride, based on the use of L-glycerol-3-phosphate oxidase (EC 1.1.3.21) and a modified Trinder's chromogen involving 4-chlorophenol, is subject to strong negative interference at concentrations of triglyceride greater than 20 mmol/L, such as occur in grossly lipemic plasma. This interference is caused by the rapid utilization of oxygen, resulting in the reaction becoming transiently anaerobic. The dye product already formed may then be reduced ("bleached") by acting as an alternative electron acceptor for glycerol-3-phosphate oxidase. Reduction of the dye leads to a marked decrease in final absorbance at 505 nm. Grossly underestimated values for triglyceride concentrations, apparently within the linear range of the assay, may therefore be inadvertently obtained with equilibrium methods. We suggest that samples giving unexpectedly low results for lipemic plasma should be re-assayed after dilution or with use of a smaller volume of sample.