Plasma thiocyanate (SCN) was measured by means of a simple colorimetric method [8] in 328 persons (175 males, 153 females, aged 5-68 years) as part of an ongoing prospective trial on the influence of SCN defined maternal smoking on fetal development. The interassay coefficient of variation for synthetic K thiocyanate solutions was 3.1% (n = 25%); the intrassay coefficient of variation was 1.2% (n = 15). Mean SCN levels were significantly lower in 226 nonsmokers (SCN 41.4 +/- 18.5 mumol/l) than 102 smokers (SCN 81.4 +/- 28.7 mumol/l; p less than 0.01). Of all the variables tested, a direct influence on SCN was found only in the case of diabetes mellitus and haemolytic blood samples (p less than 0.01), whereas age, sex, chronic diseases and nutrition had no effect on SCN. Mean maternal post partum SCN levels (n = 48) were significantly lower in non-smokers (30.2 +/- 11.2) than in smokers (59.3 +/- 23.1). There was a linear relationship between maternal venous plasma and fetal umbilical cord plasma SCN values (r = 0.88; p less than 0.01). Pregnant women had lower SCN levels than non-pregnant women. We conclude that SCN is an easy means of measuring and a reproducible parameter for quantifying smoking behaviour in pregnancy and in the general population.