Transferrin (Tf), arylesterase (ArE) and another esterase (Es) have been studied in sera from 1023 dogs by the use of isoelectric focusing (IEF) in polyacrylamide gels. Tf types were determined after protein staining in gels of pH range 5–6 and 5–7. The expression of Tf types as measured by strength of bands varied considerably. The Tf band patterns are explained by the occurrence of the 4 codominant alleles, TfF, TfM ΤfM2 and TfS of which TfM1 and TfM2are common. Some breeds had similar gene frequencies, others differed considerably. For determination of ArE types specifically stained gels of pH range 4.2–4.9 and 4.0–6.5 were employed. The ArE phenotypes appeared as multiple band patterns of which the individual bands varied considerably in strength. Atypical ArE patterns were observed in dogs suffering from certain diseases. The normal ArE phenotypes are explained by a total of 7 codominant alleles of which ArEN and ArET have not been previously described. Gene frequencies varied between breeds. For the other esterase (Es) the appearance and position of bands indicate at least 2 alleles in this system.