The high frequency of mutations in Mutator stocks of maize is the result of transposition of Mu elements. Nine different Mu elements that share the 220 bp Mu terminal inverted repeats have been described. Mu1 elements have been found inserted into most of the molecularly characterized mutant alleles isolated from Mutator stocks, and most Mutator stocks contain a high number of Mu1 elements (10-60). However, it is clear that additional Mu elements, which share the Mu1 termini but have unrelated internal sequences, can also transpose in Mutator stocks. We were interested in comparing the mutation frequency and type of elements that inserted into a particular locus when Mutator stocks with differing numbers of Mu1 elements were utilized. Furthermore, previous studies with Mu-induced mutations have demonstrated that the element that inserted most frequently was Mu1. Therefore, to try to obtain Mu elements different from Mu1 we utilized a stock that had a low number (3-6) of Mu1 elements as well as a Mutator stock with a more typical number of Mu1 elements (20-60). Utilizing both stocks, we isolated numerous mutants at one gene, Bronze 1 (Bz1), and compared the type of elements inserted. In this paper we report that both the high and low Mu1 stocks produced bz1 mutants at frequencies characteristic of Mutator stocks, 6.6 and 4.3 x 10(-5), respectively. We describe the isolation of 20 bz1 mutations, and the initial molecular characterization of eight unstable mutations: two from the high Mu1 stock and six from the low Mu1 stock. The six alleles isolated from the low Mu1 stock appear to contain deleted Mu1 elements, and the two alleles isolated from the high Mu1 stock contain elements very similar to Mu1. When the mutants from the low Mu1 stocks were examined, it was found that the Mu1-related elements increased from 3-6 copies to 9-20 copies in one generation. The high number of Mu1-related elements was maintained in subsequent outcrosses. This spontaneous activation and amplification of Mu1-related elements occurred in at least 1% of the low Mu1 plants.