The aim: Incidents with large number of casualties present a major challenge for the emergency services. Incident witnesses are always the first on scene. Authors aim at giving them an algorithm arranging the widely known first aid rules in such way, that the number of potential fatalities before the services’ arrival may be decreased. Material and methods: The authors’ main aim was creating an algorithm for mass casualty incident action, comprising elements not exceeding first aid skill level. Proceedings have been systematized, which led to creation of mass casualty incident algorithm. The analysis was based on the subject matter literature, legal acts and regulations, statistical data and author’s personal experience. Results: The analysis and synthesis of data from various sources allowed for the creation of Simple Emergency Triage (SET) algorithm. It has been proven – on theoretical level – that introducing an organized way of proceeding in mass casualty incident on the first aid level is justified. Conclusions: The SET algorithm presented in the article is of an implemental character. It may be a supplement to basic first aid skills. Algorithm may also be the starting point for further empirical research aimed at verifying its effectiveness.