Immunizing chromatin protein fractions from Drosophila melanogaster embryos, monoclonal antibodies have been generated against two nuclear proteins of different molecular weight. These proteins are present in a chromatin fraction of Drosophila Kc-cell nuclei and both proteins could be shown to cosediment with nucleosomes following separation on sucrose gradients. Early in development both proteins are located in the embryo cytoplasm. Later, at times when transcription starts at blastoderm, they become redistributed into the nuclei. On salivary gland chromosomes both proteins are detected in a series of developmentally active puffs. The number of sites where these antigens can be detected, as well as the qualitative properties of the antigens at these sites differ between both proteins.