AbstractPinocytotic vesicles are common organelles in cells of many different tissues. In the electron microscope, these vesicles appear as two types, coated and smooth. Coated vesicles have attracted recent attention with regard to the molecules involved in their formation and biochemical properties. Much less is known about smooth endocytic vesicles. In this study, we examined some of the properties of morphologically smooth vesicles in type I pneumocytes with immunological probes to coated vesicle protein components in a system where the smooth vesicle number is specifically increased by nitrogen dioxide (NO2). Using an indirect immunoperoxidase labeling technique with affinity purified anticalmodulin, anti-clathrinlight chains, and anti-clathrin-heavy chains, we were able to localize antigens on smooth vesicles that are commonly seen associated with coated vesicles by transmission electron microscopy of ultrathin sections. The labeled antigens were found on or around the vesicle membrane. In addition, results obtained with the listed primary antibodies in conjunction with an avidin-biotin-ferritin technique on thin sections embedded in Lowicryl confirmed the immunoperoxidase data. The three antigens were associated with smooth or uncoated pinocytotic vesicles. The data suggest that molecules reportedly associated with coated vesicles may also exist on smooth vesicles. It is hypothesized that some-of the molecular components that participate in the formation of coated vesicles may be involved with the formation of other vesicle types. (The J Histotechnol12:185, 1989).