This study was designed to identify and quantify concentrations of growth factors/cytokines released by Vero cells during the co-culture interval. The factors screened for in this preliminary investigation, namely platelet-derived growth factor (PDGF), transforming growth factor beta (TGFbeta), interleukin-6 (IL-6), leukaemia inhibitory factor (LIF) and epidermal growth factor (EGF) have each been identified to impact on early embryo development or are secreted by embryos themselves, suggesting an autocrine regulatory role. Vero cell culture supernatants were collected at 2, 3, 4, 5 and 6 days after seeding. Samples were assessed by enzyme-linked immunoassay for growth factor/cytokine secretion at each designated time interval. Conditioned medium from all days contained IL-6, PDGF and LIF. The concentration of IL-6 increased from 294 pg/well on day 2 to almost 1600 pg/well on day 6. PDGF also accumulated rapidly in co-culture wells, rising from 19-40 pg/well early in the culture period to around 500 pg/ well by day 6. In the second half of this study, medium supernatants from patients enrolled in our co-culture programme were analysed. Retrospective evaluation of medium supernatants collected at the time of transfer from co-cultures from 11 randomly selected patients showed considerable patient-to-patient variation in concentrations of secreted growth factors and cytokines. These findings indicate that during the co-culture interval embryos are exposed to a dynamic environment, with increasing concentrations of growth factors and cytokines. The positive effects of co-culture on embryo quality and in-vitro blastulation need to be balanced against the variation that this technique can potentially introduce into the embryo culture system.