RESUMEN
Brucellosis is a common zoonosis disease which affects both humans and animals. Common symptoms of brucellosis are: fever, sweating, weakness, lethargy, loss of appetite and arthritis. The patient was a 3.5 month infant with irritability, loss of appetite and immobility of left hip joint since 10 days ago. There was no medical problem during fetal and neonatal period of his life. The infant nutrition was provided by breast feeding. About one month ago his mother has been treated as a brucellosis patient but because of disappearance of her symptoms [pain, knee edema] she stopped her treatment ahead of schedule. In physical exam of the infant his growth and developing indices were normal but there was hypersensitivity in his left hip joint. The important laboratory results were as follows: leukocytosis, Wright=1/640, Coomb's Wright=1/1280 and 2ME=1/160. His disease diagnosed as brucellosis transmitted by lactation and he was treated with co-trimoxazole and rifampin. Breast feeding was discontinued and he was fed with formula thereafter. After a few days all symptoms disappeared and he discharged in a good condition. Applying serologic tests for infant whose mother is suspected of brucellosis seems logical