This paper deals with 1) The earlier literature dealing with the effect of steeping upon wheat flour is briefly reviewed. 2) A description is given of experiments involving the growth of eight species of mould fungi upon hard (=strong) and soft (=weak) wheat flours which had been steeped for different periods. All the species were isolated from cotton goods mildewed under industrial conditions. 3) The results as a whole support the general opinion that a period of fermentation decreases the liability of the flour to develop mildew, but they also emphasise a point which is not generally recognized. Considerable specific differences exist between the various mildew-producing fungi, that is to say, under the same conditions some species behave in a markedly different way from others. For example, as the length of the steeping period is increased, the growth of Cladosporium, a not infrequent infection of sized warps, and Fusarium is increasingly checked. In other words, steeping decreases the liability to mildew so far as these fungi are concerned. On the other hand, it has little or no effect upon the growth of Aspergillus and Penicillium, which are responsible for the greater proportion of cases of mildew trouble in size goods. In these fungi alone were considered, therefore it would appear as though steeping had no affect on liability to mildew. 4) These specific difference are also revealed when the flours are steeped with the addition of 6% zinc chloride, which decreases the liability, but much less so with some species than others. For example, the relative checking of growth of Aspergillus flavus is only one seventh that of Cladosporium. In this case the liability does not alter with the duration of steeping. 5)The addition of caustic soda to the steep until this neutral to litmus increases the liability, which is unaffected by the length of the steeping period. 6) If the steeped flour is washed free of acids, the liability is increased, but to a less extent than by neutralization. Again, the length of steeping has no effect. 7) The effect produced by adding zinc chloride to the steep is also removed by washing. 8) The chemistry of the products of fermentation is discussed in relation to the growth of mould fungi.