Recently, as part of a larger research project, we carried out a long-term teaching experiment in the ninth and tenth grades of a scientific high school aimed at getting students to approach issues of validation and of teaching of mathematical proof. Assuming a Vygotskian perspective, we focused on the social construction of knowledge and on semiotic mediation as accomplished by the teacher through the use of cultural artefacts. This paper discusses some results from the teaching experiment. It aims to clarify the role of information technologies in introducing students to a theoretical perspective. The first part of the paper introduces the construct of semiotic potential of an artefact. This construct is part of a model that was developed as result of the teaching experiment. Such a model aimed to describe the functioning of semiotic mediation in teaching – learning processes centred on the use of an artefact. The second part discusses the use of two different artefacts of information technology, a dynamic geometry environment and a software package for the teaching of algebra. The results indicate that both artefacts were successful semiotic mediators in the classroom and helped students understand how one arrives at mathematical validation.