Serum ã-glytamyl transpeptidase (GGT) and alkaline phosphatase (ALP) of the normal population in Northeastern Thailand were measured. The median serum GGP and ALP levels of females was significantly lower than the values for males, i.e., 21 (range=9-260) versus 32 (range=9-459) and 73 (range= 20-428) versus 83 (range=13-171) U/L, respectively. Serum ALP level tended to increase with age in both male and female populations, whereas serum GGT level did not show any age dependence. Serum GGT and ALP levels of cigarette smokers were significantly higher than those of the non-smokers. Alcohol, coffee or tea drinking also resulted in higher serum GGT value whereas serum ALP values were not changed. Betal nut chewing caused only lower serum GGT but not serum ALP values. The total population in Ban Fang district seemed to have lower serum GGT than those in Chonnabot district. The results from our study give the base line data of serum GGT and ALP in a Thai population. We also confirm the association of serum GGT with cigarette smoking, alcohol and coffee/tea drinking which are risk factors of cancer.