Microplastic fibres are a widespread pollutant in the marine environment. Their presence has been searched for in marine sponge specimens of a museum, collected over 20 years ago. The pollutant was observed in more than half of the samples analysed, allowing a reference point to be fixed in the past. Analysis has demonstrated that fibres were vagrant in the water column and were incorporated actively by sponges. Inclusion into bottom-fixed sponges has been demonstrated for the natural environment. The study of microplastic in organisms collected in the past and stored in natural history collections is the key for fixing reference points and build up temporal trends, especially considering the lack of studies on this topic before 1980. The idea of using animals preserved in natural history museums could be extended to other pollutants in order to search for reference points or past baselines. •Microplastic fibres debris were searched in elder sponge museum collection.•Microplastic fibres debris were found in specimens of Porifera collected more than 20 years ago.•Elder animals museum’s collections are the only way to assess past levels of microplastic debris in marine biota.•Analysing other museum’s collection would help in setting the birth date of interaction between plastic and biota.•Knowing the beginning of interaction could give useful insight to understand the effects of this harmful pollutant.