SUMMARY Hypocalcaemia resulted from the injection of grey-lethal mouse plasma into assay mice. On bioassay, parallelism in dose—response curves was established between this factor, MRC research standard calcitonin and acid extracts from the grey-lethal mouse thyroid gland. Other lines of evidence, i.e. stability at neutral and acid pH's after ultracentrifugation for long periods, after adsorption on to silica gel and the results of enzymic and acid hydrolysis, suggest that the hypocalcaemic factor is a small polypeptide. Since a hypersecretion of calcitonin could account for most of the findings in the osteopetrosis of the grey-lethal mouse it is tentatively proposed that the hypocalcaemic factor is this hormone.