A bovine cDNA library, derived from the mammary gland of a lactating cow, was screened for identifying transcripts that specifically occur during lactation by means of differential hybridisation. Several of the clones isolated by this procedure shared 55 and 57% similarity with the mouse and rat glycosylation-dependent cell adhesion molecule 1 (GlyCAM1) cDNAs, respectively. Although the mouse and cattle proteins showed an overall similarity of only 41%, two specific regions of the proteins showed 83 and 81% similarity, respectively. The bovine protein also showed 55% similarity with a small protein isolated from the whey fraction of camel milk. Northern blot analysis showed that high-level expression of this gene was only observed in the mammary gland of lactating cows. The complete gene was isolated from a bovine genomic library and its organisation was determined. The gene was 2.5 kb in length and split into four exons. The size and organization of the gene as well as the position of the introns was identical to that of the mouse GlyCAM1 gene. In accordance with the tissue-specific expression of this gene in the mammary gland of lactating animals, potential mammary gland factor (MGF) binding sites were present in the promoter region of the gene. Based on the data presented in this study it is highly likely that this gene is the bovine homologue to the rat and mouse GlyCAM1 genes.