The signs and symptoms of methylmercury poisoning and the concentrations of mercury in samples of blood, hair, and milk are reported and compared in two infant-mother pairs exposed in the recent Iraq outbreak. In one pair, the infant was born prior to the exposure of the mother, and was exposed only from ingestion of methylmercury in mother's milk. In the other pair, the mother was exposed during pregnancy and did not breast feed the infant, who died 30 days after birth. Both mothers had some signs and symptoms of poisoning, but the infants did not. The infants had maximum estimated blood mercury levels between 500 and 1,000 ng/ml, well above the minimum toxic levels for adults. Hair and blood concentrations of mercury correlated closely, the average hair-blood ratio being about 250. Maternal milk averaged 8.6% of the simultaneous blood level, but the relationship was nonlinear at blood mercury levels below 50 ng/ml. Postnatal exposure by suckling led to substantial infant blood mercury concentration (over 600 ng/ml), and prenatal exposure resulted in blood mercury levels in the infant at birth higher than those in the mother, even after corrections for differences in hematocrit level.