Pre-mRNAs cotranscriptionally associate with a small group of proteins to form heterogeneous nuclear ribonucleoprotein (hnRNP) complexes. We have previously identified two genes in Drosophila melanogaster, Hrb98DE and Hrb87F (i.e., genes at 98DE and 87F encoding putative hnRNA binding proteins), which encode five protein species homologous to the mammalian A-B hnRNP proteins. The studies presented herein show that antibodies against the RNP domains of Hrb98DE reacted with 10 to 15 distinct spots of 38 to 40 kDa in the basic region of two-dimensional gels. These nuclear proteins bound single-stranded nucleic acids and were extracted from Drosophila tissue culture cells as 40 to 80S hnRNP complexes in association with 300 to 800 nucleotide fragments of RNA. The peak of poly(A)+ RNA sequences was coincident with the peak of HRB proteins in sucrose gradients, strongly suggesting that the HRB complexes identified are Drosophila hnRNP complexes. The repertoire of HRB proteins did not change significantly during embryogenesis and was similar to that observed in Drosophila tissue culture cells. Analyses with peptide-specific antisera demonstrated that the major proteins in the hnRNP complex were encoded by the two genes previously identified. Although the Drosophila HRB proteins are only approximately 60% identical throughout the RNP domains to the mammalian A-B hnRNP proteins, features of the basic pre-mRNA packaging mechanism appear to be highly conserved between D. melanogaster and mammals.