Twenty-six Finnish Landrace × Scottish Blackface ewes, mated to a Dorset Horn ram, produced 58 lambs. Protein concentrations in the ewes' colostra during the first week of lactation, and in the ewes' and lambs' sera on the 3rd day after parturition were compared with those in pure-bred Finnish and Blackface sheep. In cross-bred and Blackface lambs the total serum protein and γ-globulin concentrations decreased significantly as litter-size increased. Litter-size had much less effect in Finnish lambs. When litters of the same size were compared, cross-bred lambs tended to have higher concentrations than Blackface lambs, but had much lower concentrations than Finnish lambs. The mean protein concentrations in sera and presuckling colostra from Finnish × Blackface ewes were almost exactly the same as in Finnish ewes, but were considerably lower and higher, respectively, than in Blackface ewes. When suckling began the concentrations in the colostra decreased much faster in Finnish ewes, even when ewes nursing the same number of lambs were compared. Differences in suckling behaviour on the day of birth may therefore partly account for the variations in the amounts of maternal protein obtained by lambs of different breeds.