ABSTRACT We generated a set of monoclonal antibodies raised against Drosophila antigen eluted from a lectin affinity column. One antibody, mAb 13D5 recognizes an anti-gen found in the most dorsal regions of the ventral mid-line of Drosophila embryos at stages prior to and during axonogenesis. 13D5 recognizes cells dorsal to the ecto-derm in the extended germ band beginning approx-imately 7 hours after fertilization and in the peripheral nervous system (PNS) at about 13 hours. In addition, 24-hour-old cultures of isolated embryonic neuroblasts possess a number of cells that express the 13D5 antigen and are not recognized by horseradish peroxidase anti-sera. These cells extend ramified processes with mult-iple growth cone-like structures or large individual processes with a broad growth cone structure. 13D5 immunoprecipitates a single band with an apparent molecular mass of 58 kDa and isolated a 1.9 kb EcoRI fragment from the Xgt11 expression libraries. In situ hybridization to staged embryos using the digoxygenin-labeled probe reveals a pattern of expression in cells just lateral to the dorsal-most regions (mesectoderm) of the ventral midline in 8-to 9-hour embryos. In situ hybrid-ization to cultured cells derived from whole embryos reveals several cell types with differing morphologies that express transcript recognized by the digoxygenin-labeled probe. These cells may possess either broad processes containing detectable transcript, or long thin processes with no detectable transcript. Northern analy-sis reveals a 2.1 kb RNA transcript detectable in all embryonic stages. Nucleotide sequence obtained from the 1.9 kb insert reveals homology with the GTP-bind-ing regions of two signal recognition particle receptors (SNRP) isolated from canine and human tissues.