VSPs are the major surface protein of Giardia lamblia and undergo antigenic variation. One VSP was purified and analyzed. The native protein contained large quantities of Zn and a lesser amount of Fe. The amino terminal sequence was cleaved off as expected and carboxyl terminal region was present indicating it likely serves as the membrane-spanning region. No carbohydrates were detected. Insulin- like growth factor is a growth factor for Giardia and stimulates growth by way of an IGF-1-like receptor on the surface of trophozoites. The receptor was mammalian-like, reacted with antibodies made to human receptors, and had intrinsic tyrosine kinase activity. This is the first demonstration of this type of receptor in an organism lower than fish and suggests unique host-parasite interactions. Lipids are essential growth factors for Giardia in serum. Some preparations also contain inhibitory substances which are likely antibodies.