Subunits A and B of cholera enterotoxin were isolated by chromatography on a Bio-Gel P-60 column in the presence of 4% formic acid. The purity and biological activity of the isolated subunits was assessed by polyacrylamide disc gel electrophoresis and mouse adrenal cell assay, respectively. The specific uptake of isolated 125I-labeled subunits A and B, peptides A1 and A2 and bovine serum albumin (BSA) by cultured adrenal cells was investigated. The results indicate that iodosubunit A, or peptide A1 or A2, traverses the plasma membrane and is released to the cell cytosol. A significant portion of bound iodosubunits A or B was associated with the plasma membrane, suggesting the presence of specific membrane receptors. The biological acitivity of subunit A was determined by the mouse adrenal cell assay. The purified subunit caused a characteristic cellular change from epithelioid to rounded morphology. A 30-fold higher concentration of subunit A (on a mole/mole basis) as compared with native toxin was required for a maximum morphologic response. These results extend previous observations related to the bioactivity of subunit A of the cholera enterotoxin molecule.