A specific, 0.1-0.3-microm large ribonucleoprotein complex consisting of a central core with stalklike extensions on top of which 280-320-A ribonucleoprotein particles are situated is found in an experimentally activated chromosome region, 2-48C, of the polytene chromosomes of Drosophila hydei. Alkaline hydrolysis, RNAse digestion, and uranyl-EDTA-lead staining indicated the ribonucleoprotein character of the 280-320-A particles, whereas the central core seems to be devoid of RNA. The characteristic complexes are present in the nucleoplasm and at the nuclear membrane, but absent from the cytoplasm. It is suggested that the large RNP complexes are the specific products of the puff at 2-48C. Complexes similar to the ones described have not been observed in any other region of the polytene salivary gland chromosomes of this species.