Human granulocyte-colony stimulating factor (hG-CSF) is a hematopoietic growth factor used in neutropenic patients. It is produced in transgenic bacteria or cultured mammalian cells. As an alternative, we now show that hG-CSF can be expressed in the mammary gland of first-generation (F1) transgenic goats during induced lactation. Despite lower milk production, transgenic females presented a similar milk composition (fat, protein and lactose) when compared to non-transgenic (p > 0.05) ones. The mean concentration (+/- SD) of recombinant hG-CSF in milk during lactation was 360 +/- A 178 A mu g ml(-1). All clinical parameters, as well as kidney and liver function, indicated that F1 transgenic goats were healthy. Additionally, no ectopic hG-CSF expression was detected in studied tissues of F1 transgenic males. Thus, F1 hG-CSF-transgenic goats can express the recombinant protein in milk at quantities compatible with their use as bioreactors in a commercial-scale protein-production program.