The content of modified albumin (Am) in harp seal (Pagophilus groenlandica Erxleben, 1777) blood serum was studied. Am was determined by paper electrophoresis by means of re-precipitation in the trichloroacetic acid-ethanol system. Modified albumin content in normal seal pups' blood serum increased from 1990 to 1994. The Am level in undernourished pups was stable from year to year and higher than in normal pups. In oceanarium investigations it was revealed a low albumin resistance to denaturation and the dependence of Am content on the animals' physiological state. Thus there is a possibility to regard modified albumin content as one of the significant parameters in biomonitoring of harp seal population and, moreover, as a supplementary criterion for estimation of seals' health state in captivity.