Fibronectin is an extracellular matrix glycoprotein. Alternative splicing of fibronectin mRNA within three specific regions, the extra domains (ED) A and B and the variable (V) or IIICS region, result in the production of different isoforms of fibronectin. These isoforms differentially regulate tissue developmental processes, such as those occurring during follicular and luteal development. The specific isoforms of fibronectin present in follicles and corpora lutea have not been identified. To identify these, primers for reverse transcription polymerase chain reaction (RT-PCR) were designed to the known bovine amino acid sequence of exons flanking the ED-A, ED-B and V regions. PCR products from bovine fetal liver cDNA were determined to be bovine fibronectin by the correct product size and DNA sequence homology to other species, and to the known bovine amino acid sequence. Bovine ovarian follicles (0.5-9 mm diameter) and corpora lutea (cyclic, early to late mid-luteal phase) were shown to express ED-A+, ED-A-, ED-B+, ED-B-, V+ and V fibronectin isoforms, similar to the liver, lung and kidney of fetuses, but generally not of adult animals. Thus follicles and corpora lutea express isoforms of fibronectin usually expressed in developing tissues.