Hemolin is a protein from the immunoglobulin (Ig) superfamily found so far in the haemolymph of two lepidopteran insect species, Hyalophora cecropia and Manduca sexta. Injection of bacterial into these insects induces the expression of hemolin. We have isolated the hemolin gene from M. sexta and determined its DNA sequence and transcription start site. The hemolin gene is 3127 bp long and contains six exons. The only correspondence between exons and the four Ig domains of hemolin is in domain 4, which is encoded by exon 6. Southern blot analysis indicates that there is one copy of the hemolin gene in the M. sexta genome. Analysis of the 5'-flanking sequence of the hemolin gene resulted in identification of potential regulatory sequences. Hemolin mRNA accumulated in haemocytes, as well as fat body, in response to injection of larvae with bacteria. Hemolin was detected by immunocytochemistry in only one of the five morphological haemocyte types in M. sexta, the granular cells.