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Reasons for performing study Multipotent mesenchymal stromal cells ( MSC) are often culture-expanded in vitro. Presently, expansion medium ( EM) for MSC is supplemented with fetal bovine serum ( FBS). However, increasing cost, variable composition and potential risks associated with bovine antigens call for alternatives. Platelet lysate ( PL) has shown promise as an alternative supplement. Objectives To determine how equine umbilical cord blood ( CB) MSC proliferate in EM enriched with PL or FBS at various concentrations. Study design Randomised dose escalation study. Methods Platelet concentrate was generated from 5 equine whole blood samples through a double centrifugation method and standardised to 1 × 1012 platelets/l prior to a freeze/thaw cycle to produce PL. Pooled PL or pooled FBS was added to EM at concentrations of 5% to 60%. Proliferation of 4 equine CB- MSC cultures was determined after 4 days using a resazurin semiquantitative assay. Results Cord blood- MSC proliferated with a dose-dependent response with no significant difference found between PL and FBS up to a 30% concentration. Beyond 30%, proliferation fell in the PL-cultured cells, while continued dose-dependent proliferation was noted in the FBS-cultured cells. Despite reduced cell numbers in high PL concentrations, live/dead staining revealed that adherent cells remained viable. Conclusions Expansion medium enriched with PL can support short-term equine CB- MSC proliferation at conventional culture concentrations. Based on the unexpected suppression of CB- MSC at higher PL concentrations, an in vivo dose study is indicated to investigate if combinational therapies of CB- MSC and platelet-rich plasma are associated with synergistic or antagonistic effect on CB- MSC function. [ABSTRACT FROM AUTHOR]