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Transgenic animals secreting individual chains and assembled ﬁbrinogen were produced to evaluate the capacity of the mammary gland for maximizing assembly, glycosylation and secretion of recombinant human ﬁbrinogen (rhﬁb). Transgenes were constructed from the 4.1 kbp murine Whey Acidic Protein promoter (mWAP) and the three cDNAs coding for the Aα,Bβ and γ ﬁbrinogen chains. Transgenic mice secreted fully assembled ﬁbrinogen into milk at concentrations between 10 and 200 μg/ml, with total secretion of subunits approaching 700 μ g/ml in milk. Partially puriﬁed ﬁbrinogen was shown to form a visible and stable clot after treatment with human thrombin and factor XIII. The level of assembled ﬁbrinogen was proportional to the lowest amount of subunit produced where both the Bβ and γ chains were rate limiting. Both the Bβ and γ chains were glycosylated when co-expressed and the degree of saccharide maturation was dependent on expression level, with processing preferred for γ chains over chains. Also, the subunit complexes Aαγ2, γ2 and the individual subunits Aα, Bβ and γ were found as secretion products. When the Bβ was secreted individually, the glycosylation proﬁle of the molecule was of a mature complex saccharide indicating recognition of the molecule by the glycosylation pathway without association with other ﬁbrinogen chains. To date secretion of Bβ chain has been not observed in any cell type, suggesting that the secretion pathway in mammary epithelia is less restrictive than that occurring in hepatocytes and other cells previously used to study ﬁbrinogen assembly.