Semaphorins are a large family of secreted and transmembrane proteins required for guidance of pioneering axons to their predestined targets during neuronal development. Acting as inhibitors, chemo-repellents or attractants, semaphorins appear to affect axon steering, fasciculation, branching, and synapse formation. However, research into the function of semaphorins in non-neuronal systems has highlighted the important roles these molecules play in the immune system as well as in organogenesis, vascularisation and angiogenesis. Semaphorins have also been implicated in neurodegenerative diseases such as Parkinson's disease, and in the progression of certain types of cancer. The role of the leukocyte semaphorin, semaphorin4D (Sema4D) and its interaction with receptors are the focus of these studies. Sema4D is a 150 kDa transmembrane protein expressed on the surface of most haemopoietic cells including B and T lymphocytes. It is reportedly involved in T-cell activation and B-cell aggregation and survival, via the c-lectin, CD72. It is one of only a few semaphorin family members known to play a physiological role in the immune system. In contrast, Sema4D is also able to repel projecting axons through an interaction with PlexinB1, suggesting that it also plays an important role in the developing neuronal network. Studies described here are concerned with the structure of Sema4D and initial studies on its PlexinB1 receptor. Plexins, like semaphorins, contain a sema domain and usually 3 PSI domains. Having determined the structure of Sema4D, studies are underway to solve the structure PlexinB1, and the Sema4D-PlexinB1 complex. Aside from the structural studies, the functions of several aspects of Sema4D are of interest. These include, the specific role of the PSI domain which is also present in both plexins and integrins; mapping of the Sema4D-plexinB1 binding region using site-directed mutagenesis and Biacore binding studies; and a search for other molecules that interact with Sema4D.