In order to maximize the mutagenic effectiveness of N-ethyl-N-nitrosourea in 
mouse stem-cell spermatogonia, advantage was taken of the fact that these cells 
can accumulate mutations from repeated doses given over relatively long time 
periods. Repeated doses (100 mg/kg) of ethylnitrosourea injected 
intraperitoneally into male mice at weekly intervals were found to allow 
adequate survival and fertility with total dosages of 300 and 400 mg/kg. The 
specific-locus mutation frequencies at these dosages were, respectively, 1.8 and 
2.2 times that obtained with the maximal practicable single dose of 250 mg/kg. 
The mutation frequency induced by a 400 mg/kg dosage of ethylnitrosourea is 12 
times the maximal mutation frequency achievable with a single exposure to x-rays 
and 36 times that reported for procarbazine, the most effective chemical mutagen 
previously known for mouse stem-cell spermatogonia. Ethylnitrosourea is already 
the mutagen of choice in deliberate attempts to create mouse models for human 
disease and in any experiments in which a maximal mutation rate is desired. 
Repeated-dose regimens similar to the ones reported here should increase the 
efficiency of such studies.