Spontaneous mutant frequency of lacZ gene in spleen of transgenic MutaTM mouse was examined at different ages. It was (3.2 +/- 1.3 (SD)) x 10(-5) at newborn and increased almost linearly with age up to (8.3 +/- 1.8) x 10(-5) at one year. Since the mutation of the gene is not likely to be subject to selection in vivo, the data support the idea that spontaneous mutation takes place throughout aging process and accumulates with age if not selected out by cell death.