A previous study was undertaken to test the reaction of several quinones (p-benzoquinone; 2,5-dichloro and 2,6-dichloro p-benzoquinone; tetrachloro-p-benzoquinone; tetrachloro-o-benzoquinone; 2,5-dichloro-3,6-dihydroxy-p-benzoquinone; benz[a]anthracene-7,12-dione) with bovine serum albumin (BSA). From this study, we have devised a spectrophotometric method for determination of total proteins. The quinone, tetrachloro-p-benzoquinone (p-chloranil), showed the best result. The product of reaction between proteins and p-chloranil absorbed at 360 nm and Beer's law was followed up to 200 mug ml(-1) of BSA. The product of reaction of BSA/p-chloranil was stable for 30 min, after that the absorbance increased 16% and kept stable for 24 h. The p-chloranil method showed a limit of detection (1.25 mug ml(-1)) lower than the biuret method (52.0 mug ml(-1)) or p-benzoquinone (PBQ) method (2.6-4.0 mug ml(-1)). The method was applied to spectrophotometric determination of total proteins in blood plasma; the results were compared with the biuret method that is widely used in clinical analysis.