A HeLa cell/neutral red cytotoxicity assay has been investigated as an in vitro screen for assessing the irritancy potential of parenteral formulations. Five commercially available intramuscular injectables, three irritants and two non-irritants, were examined for cytotoxic effects in this system. The ranking of the injectables tested, in order of decreasing cytotoxic effects, correlated with both previously reported qualitative in vitro data and their reported irritancy when administered to man. The results show that the HeLa cell/neutral red cytotoxicity assay may provide an alternative to animal studies for the assessment of the irritancy potential of parenteral formulations during the preliminary stages of their development.