We have characterized and cloned the lactation‐specific mRNAs of mouse mammary glands. The group of eight milk‐protein‐specific mRNAs were identified (a) by size and antigenic properties of their translation products in vitro and (b) by characterization of their respective cDNA clones.Two α‐caseins (43 kDa and 39 kDa) are encoded by mRNAs of 1600 nucleotides and two β‐caseins (26 kDa) are encoded by mRNAs of 1450 nucleotides in length. Three smaller caseins, γ‐casein (23.7 kDa), δcasein (21 kDa) and £‐casein (14.5 kDa) are synthesized by mRNAs of 880, 1150 and 860 nucleotides. Beside these casein mRNAs a mammary specific 620 nucleotide mRNA codes for a novel acidic whey protein (13.7 kDa).cDNA clones corresponding to the mRNAs for the lactation‐specific proteins have been isolated from a mammary‐specific cDNA library. Cloned α‐casein cDNA hybridizes to both β‐casein specific mRNAs and cloned β‐casein cDNAs hybridize with both β‐casein specific mRNAs. By RNA blot analysis we show that the cloned β ‐casein cDNAs for mouse α‐casein, β‐casein, γ‐casein and &‐casein and the acidic whey protein cross‐hybridize with mRNAs of the rat, demonstrating partial sequence homology between the corresponding mRNAs of those species.