Male mice were treated with graded doses of methyl methanesulfonate (MMS) and mated with untreated female mice during the six to ten days after treatment. The nature of the primary lesions induced in late spermatids was investigated by analysing the types and rates of chromosome aberrations in the first cleavage metaphases. The chromosome aberrations recovered at the first cleavage were predominantly of the chromosome-type. Their frequency increased exponentially with increasing dose of MMS. The results suggested that the nonenzymatic conversion of DNA containing alkylated bases to new damage, possibly strand breaks, during the maturation and storage of sperm was primarily responsible for the enhancement of the frequency of chromosome-type aberrations.