Specific locus (gol-1) germ line mutations are induced by gamma-rays with high frequencies (about 10(-5) r( -1)) during cleavage divisions in zebrafish. Mutant clone sizes range from 3 to 50% of the total number of germ cells, with a mean of about 10%, when embryos are exposed between the 16 and 10(3) cell stages. About five pregonial cells are calculated to be present during the cleavage period.