To obtain specific immunological probes for studying molecular events involved in gene activation during early development of Drosophila, we have produced monoclonal antibodies directed against nuclear proteins differentially expressed in time and space during embryogenesis. Twenty‐five antibodies against nuclear antigens detected after the onset of zygotic genome transcription were obtained. These antigens have been distributed into six categories depending on their reactivity in dot‐blot, in Western‐blot and in immunofluorescence on embryo frozen section assays. Six antigens (fifth and sixth categories) are stage‐, but not tissue‐specific and are present in all nuclei of the embryo after the blastoderm stage. Ten antigens, the third and fourth categories, did not react on immunofluorescence and have not been characterized for a possible tissue specificity. Antigens of the first and second categories are stage‐ and tissue‐ specific. Two of them are of particular interest. The first (250 kd, recognized by the monoclonal antibody GB7) is preferentially expressed in nuclei of the ventral nerve cord, whereas the other (65 kd, recognized by the monoclonal antibody LA9) is located in nuclei of the gut and associat