In the present study an attempt has been made to characterize the genetic ``factors'' controlling quantitative characters, bristle numbers, in Drosophila melanogaster. A low sternopleural bristle multiple recessive marker third chromosome was used to analyze a high sternopleural third chromosome, in a high sternopleural bristle background. An attempt was made to estimate the minimum number of ``effective factors'' involved in the difference in bristle score between the tested and marker chromosomes. Apart from sternopleural, scutellar and ocellar bristles, a new character, subprimal bristles, was also scored. The unselected characters were used to help in the factor locations, and an attempt made to detect epistasis. Concentrations of bristle effects were found, as were a few `factors' of large effect. At least 17 sternopleural bristle factors are required to account for the difference in bristle score between the high tested third chromosome and the low tester third chromosome. There was an ascertainment problem for polygenes with effects of less than about 0.6 phenotypic standard deviation. Only an estimate of the minimum number of factors and approximate locations can be given with any degree of certainty. The results are compatible with the hypothesis (among others) that quantitative characters are under the control of a few major genes supported by numerous genes with smaller effect.