Retinyl beta-glucuronide has been identified as a normal endogenous component of human blood, as have retinoyl beta-glucuronide and retinoic acid. The mean serum concentration of retinyl beta-glucuronide in adult volunteers (n = 6) was 6.8 +/- 4.0 nmol/L (means +/- SD) with a range from 1 to 11 nmol/L. Purified serum retinyl beta-glucuronide was characterized by the mass spectrum of its methylated trimethylsilyl derivative, by its hydrolysis by beta-glucuronidase, and by its chromatographic behavior in high-performance liquid chromatography systems. During a single day, concentrations of retinyl beta-glucuronide, retinoyl beta-glucuronide, and retinoic acid varied approximately two-fold.