Detailed restriction enzyme maps have been determined for the three major rat casein and the fourth principal milk protein, alpha-lactalbumin, cDNA clones. Each of the milk protein genes exhibited unique and characteristic restriction enzyme sites. A comparison of the restriction enzyme maps of the three rat caseins revealed no apparent sequence homology among their gene sequences. The orientation of each cDNA gene sequence within the parent plasmid, pBR322, was determined by hybridization with a 3' specific cDNA probe synthesized from a partially hydrolyzed total poly(A) mRNA preparation following isolation by chromatography on oligo(dT)-cellulose. This technique provided a rapid procedure for determining the 5'-3' orientation of the cloned DNA sequences. Three casein clones were selected, which were in the same orientation, and were employed for a heteroduplex analysis to determine whether minor regions of homology existed within the alpha-, beta-, and gamma-casein genes. No heteroduplex formation was observed among these genes even under the low stringency conditions of hybridization employed, suggesting that considerable sequence divergence has occurred within the rat casein gene family.