Methods for determining the viability of an embryo or an embryo precursor, comprising measuring the level of a polypeptide or peptide released by a cultured embryo or embryo precursor into the culture medium are disclosed. The polypeptide may be p53. Also disclosed are methods of ranking embryos on the basis of their viability using such methods. Methods for screening for agents which modulate embryo viability, as measured by changes in the level of a polypeptide marker released by a cultured embryo or embryo precursor are also described.