Currently, radiotherapy is a cornerstone treatment for pelvic tumours. Unfortunately, it involves exposure to very high doses up to 50 Gy cumulated dose (Peters 2000) causing damage to healthy surrounding gastrointestinal (GI) tissue, which is rarely the primary target yet always affected. Moreover, microbial dysbiosis develops leading to diarrhoea and fatigue in 50% of the patients. These symptoms can severely worsen and might require interruption or additional treatments (Touchefeu 2014; Kim 2015). Increasing evidence states that the human microbiome influences radiation induced health effects (Packey and Ciorba 2010). As a result, bolstering of the human microbiome has become a hot research topic in various medical disciplines with the succesful use of pre- and probiotics (eg. Lactobacilli and Bifidobacteria) to enhance nutrition and health (Delia 2007; Ciorba & Stenson 2009; Johnke 2014). To counteract GI radiotoxicity, novel microbial therapies are being developed such as VSL#3 and L. rhamnosus GG (LGG), which have shown promising results in treating diarrhoea. Despite its easy, cheap, safe and feasible approach to protect patients against acute radiation induced diarrhoea (Ciorba 2015; Johnson & Klaenhammer 2014), trials have yielded contradictory results. We therefore present the edible photosynthetic cyanobacterium Arthrospira sp. as a very promising, health promoting food supplement that has been used for centuries because of its high nutritional content of proteins, minerals and vitamins. For instance, it has been used to boost the immune system of children exposed to radiation after the Chernobyl disaster and was reported to exibit neuroprotective, immunomodulatory and anticancer activities (Yogianti 2014) next to strong anti-oxidant and metal binding capacities. Furthermore, its radioprotective potential was already reported by Qishen in 1989 in murine bone marrow cells. SCK•CEN has recently demonstrated Arthrospira sp. PCC8005’s radiation resistance, surviving doses of 6400 Gy (Badri 2015). However, the additional capacity of Arthrospira sp. as a food supplement to ensure protection and/or faster recovery from radiation damage to human subjects remains to be proven. The aim of this project is to explore and characterise the potential of Arthrospira sp. microbial therapy to prevent pelvic irradiation induced GI toxicity in vivo and to compare this data with LGG’s radioprotective capacities.