It is important that the instrument used to provide estimates of the blood alcohol concentrations (BAC) of individuals, in this case the Intoxilyzer 5000C, be properly calibrated. The calibration of the Intoxilyzer 5000C involves the use of an aqueous alcohol standard; the current study assessed the ability of this alcohol standard to accurately and reliably assess the calibration of the Intoxilyzer 5000C while in extensive use in the field. Seven Intoxilyzer 5000C were operated in re-circulating mode for 12 days during a training course. The Intoxilyzers had been connected to a Guth Laboratory Model 34 C simulator filled with 500 mL of alcohol standard solution. A minimum of 25 calibration checks were conducted on each instrument on days 2 through 5 and days 8 through 11 of the study, resulting in an average of 218 calibration checks per instrument. The calibration checks resulted in a 5 percent decrease in the concentration of the alcohol standard. While a decrease was noted in the mean of the calibration check results, 99.7 percent of the calibration check results were within the acceptable range of 90 to 110 mg/100mL, showing that the instrument was calibrated. The findings thus indicate that the aqueous alcohol standard is suitable for the assessment of the calibration of the Intoxilyzer 5000C during extensive field use. Tables, references