Lasalocid is a coccidiostat licensed for use in poultry, but not for use in egg‐laying birds. Lasalocid residues were determined in an egg sample from each of 161 egg producers in Northern Ireland using liquid chromatography‐electrospray ionization mass spectrometry, following reports from the Veterinary Medicines Directorate concerning the incidence of lasalocid residues in the United Kingdom. Approximately 66% of the eggs contained lasalocid residues at concentrations in excess of 0.3 ng/g. There was no apparent difference in the incidence of lasalocid residues between free range and battery eggs. Carry‐over of lasalocid from medicated to unmedicated batches of both premix and feed, during milling processes, was identified as a possible cause of contamination. Subsequently, egg‐laying birds were fed meal containing a range of lasalocid concentrations, similar to those found as a result of unintentional contamination at a feed mill (0.1–5.0 mg/kg). The concentrations of lasalocid, measured in their eggs, were similar to that found in the survey. Lasalocid persisted in eggs for 10 days after withdrawal of medicated feed and replacement with lasalocid‐free feed.