Smooth muscle is composed of cigar-shaped, non-striated cells, each of which is encapsulated by a basement membrane and forms the contractile portion of tubular organs such as the gastrointestinal tract, pulmonary tract, genitourinary tract, and vasculature, in which slow and sustained contractions are needed. We examined basement membranes produced by smooth muscle cells and, using alpha(IV) chain-specific monoclonal antibodies, analyzed type IV collagens in these organs. Detailed distribution analysis of the alpha chains in normal and Alport cases by use of specific antibodies indicated that there are at least three molecular forms of type IV collagen, [alpha1(IV)]2alpha2(IV),alpha3(IV)alpha4(IV)alpha5+ ++(IV), and alpha5(IV)/alpha6(IV). Smooth muscle cells in the urinary bladder and uterus were enclosed by basement membranes composed of alpha1, alpha2, alpha5, and alpha6 chains. The same alpha chains were present around smooth muscle cells in the muscular layer of the fundus of the stomach, whereas those in the antrum and further distal side of the gastrointestinal tract expressed mostly alphal and alpha2 chains. In addition, immunostaining analysis of the vasculature also showed that most of the smooth muscle cells were positive for alpha1 and alpha2 chains; however, alpha5 and alpha6 chains were also expressed by smooth muscle cells in the aorta and some arteries where blood pressure changes significantly. These results suggest that the smooth muscle cells enclosed by alpha5/alpha6-containing basement membranes might have some particular function related to mechanical stress or tensile strength during the characteristic contractile activity of tubular organs.