The protein moiety of duck globin messenger ribonucleoprotein complexes isolated by oligo(dT)-cellulose chromatography or by sucrose gradient centrifugation was analysed by two-dimensional polyacrylamide gel electrophoresis under conditions where the separation in the first dimension occurs according to charge and in the second according to molecular weight. By comparing the pattern of protein from the mRNA - protein complex with that of ribosomal subunits we found that two acidic proteins with an identical molecular weight of about 49 000 and three basic proteins of about Mr 56 000, 64 000 and 73 000 were associated with the duck globin mRNA but were absent from either puromycin/high-salt-derived or 'run-off' ribosomal subunits. The comparison of the proteins from the complex with mRNA with those found in the 0.5 M KCl wash, commonly used as the source of initiation factors, showed also that only the 49 000-Mr protein from the complex could possibly be present in the 0.5 M KCl wash of polyribosomes; proteins with mobilities similar to the other three proteins complexed with mRNA were not detected in the salt wash of polyribosomes.