A quite specific kind of sterile F1 female, called SF females, arises only when females of strains denoted reactive are crossed with males of the other class (inducer). It was previously shown that this sterility results from a nucleocytoplasmic interaction between the maternal reactive cytoplasm and a factor, called I, which may be born by any one of the paternal chromosomes. In SF females, but not in their brothers, a varying proportion of reactive chromosomes are able to acquire irreversibly the I factor, independently of any classical genetic recombination with the inducer chromosome(s). During this process, called chromosomal contamination, the contaminating chromosome(s) do not undergo any apparent change. The present paper deals with the efficiency of both original inducer and contaminated chromosomes to yield a more or less intense SF sterility. The Otanu inducer laboratory strain contains at least two types of X chromosomes (called strong and weak) which differ genetically with respect to their inducer efficiency. Reactive third chromosomes were contaminated by these strong or weak X chromosomes and their inducer efficiencies compared. Results show that they are on average stronger when they have been contaminated by strong X chromosomes than when contaminated by weak ones. Such a correlation favours the hypothesis that chromosomal contamination is due to the insertion of some genetic element(s) into reactive chromosomes.