The globin mRNAs containing between 30 and 40 polyadenylate residues can be separated from those mRNAs containing longer poly(A) regions by Millipore filter binding. The molecular weights of the α- and β-globin mRNAs containing this size class of poly(A) have been determined by electrophoresis on 3.7% polyacrylamide gels in the presence of 99% formamide. Because the number of adenylic acid residues in these mRNAs is known, the number of non-poly(A) nucleotides can be accurately calculated. The molecular weight of the β-globin mRNA is 235 000 ± 28 000 ( 736 ± 88 nucleotides ) and that of the α-globin mRNA is 208 900 ± 43 870 ( 653 ± 78 nucleotides ). By subtracting the number of nucleotides in the coding and poly(A) regions, the number of noncoding nucleotides in the β-globin mRNA were calculated to be 261, 68 more than the 193 present in the α-globin mRNA. Comparison of size estimates of newly synthesized globin mRNAs containing longer average lengths of poly(A) showed that there is no comparable processing of the 5′ termini of the α- and β-globin mRNAs concomitant with the stepwise degradation of the poly(A) regions which occur as the mRNAs mature.