Cultures of dissociated hippocampal neurons are often used to study neuronal cell biology. We report that the development of these neurons is strongly affected by chemicals leaching from commonly used disposable medical-grade syringes and syringe filters. Contamination of culture medium by bioactive substance(s) from syringes and filters occurred with multiple manufacturing lots and filter types under normal use conditions and resulted in changes to neurite growth, axon formation and the neuronal microtubule cytoskeleton. The effects on neuronal morphology were concentration-dependent and significant effects were detected even after substantial dilution of the contaminated medium. Gas chromatography-mass spectrometry analyses revealed many chemicals eluting from the syringes and filters. Three of these chemicals (stearic acid, palmitic acid and 1,2-ethanediol monoacetate) were tested but showed no effects on neurite growth. Similar changes in neuronal morphology were seen with high concentrations of bisphenol A and dibutyl phthalate, two hormonally active plasticisers. Although no such compounds were detected by gas chromatography–mass spectrometry, unknown plasticisers in leachates may affect neurites. This is the first study to show that leachates from laboratory consumables can alter the growth of cultured hippocampal neurons. We highlight important considerations to ensure leachate contamination does not compromise cell biology experiments.