Genotoxic bystander signals released from irradiated human mesenchymal stromal cells (MSC) may induce radiation-induced bystander effects (RIBE) in human hematopoietic stem and progenitor cells (HSPC) potentially causing leukemic transformation. Although the source of bystander signals is evident, the identification and characterization of these signals is challenging. Here, RIBE were analyzed in human CD34+ cells cultivated in fractions of filtered medium conditioned by 2 Gy irradiated human MSC. Specifically, γH2AX foci (as a marker of DNA double strand-breaks) and chromosomal instability (CIN) were evaluated in CD34+ cells grown for 3 days in (a) < 10 kDa, (b) 10 – 100 kDa and (c) > 100 kDa fractions of MSC conditioned medium and un-/fractionated control medium, respectively. Hitherto, substantial bystander effects were detected predominantly in CD34+ cells grown in 10 – 100 kDa fractions of MSC conditioned medium when compared to < 10 kDa or > 100 kDa fractions of MSC conditioned medium or fractionated control medium. Taken together, our data suggest that RIBE are predominantly mediated by the 10 – 100 kDa fractions of conditioned medium. This finding might be important for the identification of key bystander signals by in depth proteome analysis, which might contribute to the development of next-generation anti-leukemic drugs.