Thirteen infants born into the lead contaminated environment of Port Pirie, South Australia, were followed approximately monthly from birth until they were about 36 months. Blood-lead levels of infants at birth were similar to their mothers but fell rapidly during the first 35 days of life. Thereafter, infants born with blood-lead levels at about 2-4 microg/dl began a slow linear increase until 14-18 months where a plateau occurred of 10.8-17.2 microg/dl. The blood-lead levels were well correlated with hand-lead loadings of infant (r(2)=0.72, P<0.01, log transformed data) and mother (r(2)=0.62, P<0.01, log transformed data) unless the birth lead level was exceptionally high. The principle factor determining exposure was the impact of smelter emissions on the house. Blood-lead increase was caused by the relatively more rapid increase in dose of lead compared with the increasing body mass, which was related directly to the maturation of motor development. Hand-lead of mothers were closely related to both infants' blood- and hand-lead levels until the point of blood-lead plateau then substantially fell as infants began to walk unaided. The estimated slope factor using the ICRP model was 0.75-0.94 microg/dl per microg/day with a maximum daily dose of 3-5 microg/kg/day, assuming 45% absorption. Ingestion appears to be the most likely route for at least 95% of the dose.