The quantitative analysis of coenzyme Q10 (CoQ10) in samples of whole human blood has been refined to allow a 2- to 3-fold increase in the number of analyses per day, and reduction of cost to approximately 15% of the previous cost. The method is simple yet maintains reliability. The standard error was 0.2% (n = 6). The variation in blood levels of CoQ10 for human subjects for each of three months was approximately 5% in comparison with the control value (n = 5). For 30 human males, of 18-50 years (26 +/- 6) in age, and for 30 human females, of 18-50 years (26 +/- 9), the mean blood level of CoQ10 was 0.71 +/- 0.13 microgram/ml and 0.70 +/- 0.18 microgram/ml respectively. The mean blood levels of CoQ10 of rabbits (n = 28) was 0.29 +/- 0.07 micrograms/ml, and that for rats (n = 29) was 0.23 +/- 0.03 micrograms/ml.