Spermatozoa without acrosomes ("acrosomeless") have been qualitatively and quantitatively analysed in several strains of mouse. The frequency of these abnormal cells was fairly constant within the same strain, but increased with age, consanguinity and after treatment of mice at pre-meiotic stages with alkylating agents. From this behaviour, it is thought that these abnormal cells are true mutants which could be used in mutation research.