The Abdominal-B (Abd-B) gene, a member of the Drosophila bithorax complex, is required during development to specify the identity of parasegments 10-14. Based on genetic studies, Casanova, J., Sánchez-Herrero, E. and Morata, G. (1986) Cell 47, 627-636, proposed that the Abd-B gene consists of two distinct elements that provide a morphogenetic (m) function in PS 10-13 and a regulatory (r) function in PS 14, where it represses m function. Here we present molecular confirmation of this genetic model. Using specific antibodies, we show that the 55 X 10(3) M(r) and 30 X 10(3) M(r) Abd-B proteins, predicted by cDNA analysis, are indeed present in PS 10-13 and PS 14, respectively. We also examine Abd-B mRNA and protein expression patterns in embryos mutant for either the m or r function. These data allow us to unambiguously assign m function to the 55 X 10(3) M(r) protein and r function to the 30 X 10(3) M(r) protein. Furthermore, as postulated by the model, transcription of the mRNA encoding the m protein is derepressed in PS 14 in the absence of r function. We have also studied the effect of mutations mapping in the infra-abdominal (iab) region located downstream of the Abd-B gene. Genetic studies suggest that the iab region contains cis-acting regulatory elements controlling Abd-B expression in PS 10-12. We present molecular evidence for the presence of downstream cis-regulatory elements by analyzing Abd-B mRNA and protein patterns in iab-6 and iab-7 embryos. Our analysis reveals the presence of parasegment and cell-specific regulatory elements of the Abd-B gene within each iab region. The Abd-B gene may provide a model for the understanding of similarly complex homeotic genes in higher organisms.