Coculture studies using preimplantation embryos have led to a number of conflicting studies. In the human, ethical considerations have led to the preferential use of epithelial cell lines as distinct from human Fallopian tube cells. In an attempt to isolate factors influencing embryo development we have cultured 2-cell OF1 mouse embryos in media [Ménézo's B2 and Whittingham's T6 supplemented with vitamins and amino acids (T6VA)] conditioned on two types of kidney epithelial cells (MBDK and Vero). Different molecular weight fractions of conditioned medium were used to show the absence or presence of specific embryotrophic factors. With MDBK cells, B2 conditioned medium enhanced embryo development up to the blastocyst stage, while no blastocysts developed in B2 alone. When using T6VA medium, both the control and conditioned media showed a high percentage of blastocyst formation (57.0 and 54.0% respectively), while the different molecular weight fractions showed no added improvement. With Vero cells, B2 alone, B2 conditioned medium and fractions were all detrimental to embryo development. A high percentage of blastocyst formation (between 64.7 and 75.8%) was observed in T6VA alone. T6VA conditioned medium and fractions. Low blastocyst formation in a control medium can show strong positive results when medium is conditioned by cells. In contrast, a good base medium, such as T6VA, can equal the results using conditioned medium. Different cells in contact with different types of medium show variability in the pattern of responses, highlighting the presence of false positives in coculture studies.