We have developed an expression system where foreign proteins are synthesized specifically in the mammary gland of transgenic rabbits and secreted into the milk. Regulatory elements were isolated from the bovine alpha(S1)-casein-encoding gene and combined with a synthetic DNA coding for human IGF-1 and for [GIn(58)]IGF-1, an IGF-1 analogue. The resulting hybrid DNA constructs were used to generate transgenic rabbits. Females of seven transgenic lines tested were positive for synthesis of IGF-1. Transmission of the transgene to progeny and IGF-I production in female offspring was observed in all transgenic lines analysed. As expected, expression of transgene mRNA could only be detected in the mammary gland. Production levels of transgenic protein were as high as 1 g IGF-1 per liter rabbit milk. IGF-1, as well as [Gln(58)]IGF-1, when secreted into rabbit milk, was correctly processed and biologically active. IGF-1 was purified from the milk of transgenic rabbits to a nearly homogenous active form.