Purpose: The purpose of this study is to understand the influence of formal learning from a web-based training and informal (workplace) learning afterwards on the behaviour of general practitioners (GPs) with respect to prescription of antibiotics. Design/methodology/approach: To obtain insight in various learning processes, semi-structured interviews were conducted with 19 GPs. These interviews were transcribed and analysed with a theory-based template, that had been defined beforehand, but with an open mind for emerging themes. Findings: The web-based training was perceived by GPs to change their prescription behaviour, mostly as a result of informal learning processes. Being a research participant and being a supervisor appeared to create most opportunities for informal learning. Practical implications: The current research shows that being a research participant and/or a supervisor enhance informal learning activities, for example, reflection and social interaction, and thereby formal training becomes more effective. It is recommended to remind GPs regularly to reflect on their prescribing behaviour and to stimulate them to reflect and seek social interaction besides participating in formal training. Originality/value: Our study adds to the existing literature by considering informal learning processes in an evaluation of the perceived effects of formal training. Our findings have implications for the design and evaluation of formal trainings with the purpose of behavioural change of doctors.