The authors measured the concentrations of serum thiocyanate (SCN) of 80 infants in Chang-Ning District, Shanghai, People's Republic of China. The infants were classified into one of three groups according to the total number of cigarettes family members collectively smoked per day: (1) unexposed, (2) lightly exposed (1-19 cigarettes/d), and (3) heavily exposed (20+ cigarettes/d). The serum SCN levels (mean +/- SD, mumol/l) were 27.7 +/- 10.72 for the unexposed group, 31.9 +/- 13.5 for the lightly exposed group, and 36.2 +/- 14.88 for the heavily exposed group. The concentrations in the heavily exposed group were significantly higher than those in the unexposed group (Z = 2.12, p less than .05). Environmental exposure to tobacco smoke accounted for 5.3% of total variance of the SCN levels after adjusting the effects of father's education level and type of feeding. It is concluded that passive exposure to cigarette smoke in the household results in higher levels of serum SCN in infants.