The level of adenosine deaminase (EC. 3.5.4.4), was estimated in plasma of 389 healthy males and 493 healthy females in order to establish a normal reference range for Saudis. Using the continuous spectrophotometric method, the reference ranges were calculated in two ways using the mean ± 2 SD and the 2.5th -97.5th percentile value methods. In both methods of calculation, a slightly higher range was observed for children as compared to adults. The method of 2.5th -97.5th percentile values brought almost all of our subjects within the recommended range of 11.5 -25 U/1. In the current study, the normal range for adenosine deaminase totalled 15.0 -23.2,14.8 -23.6,15.0 -23.0 and 16.7 -24.6 U/1 for the overall population, all males, females, and children, respectively. The ranges are discussed in the light of significantly different results obtained by the two calculation methods and recommendation of an appropriate method for healthy Saudis, namely the 2.5th -97.5th percentile values. The choice of the Ellis and Goldberg kinetic continuous monitoring method for the estimation of plasma ADA levels in the current investigation is also hereby justified.FJ. Al-Shammary, CC. Anokute, HK Ghneim, AM Na'seh, AD Al-Wazzan, SA Al-Majed,