Pooled or individual plasmas from normal men, women, pregnant women (third trimester), anephric women, and rat liver perfusates were used as sources of angiotensinogen. The plasmas were fractionated and desalted by Sephadex gel filtration, then subjected to isoelectric focusing in a pH 4 to 6 gradient on 40 X 4 cm slabs of polyacrylamide gel. The gels were cut transversely into 0.5-cm-wide strips, the pH measured, and their angiotensinogen concentrations determined by incubation with excess human renin and radioimmunoassay of the product, angiotensin I. This revealed several peaks of angiotensinogen concentration indicative of microheterogeneity in all cases. Contrary to other claims, the isoelectric pH profiles of angiotensinogens in the various physiological states were substantially alike. Major peaks were found at pH 4.75 to 4.85 and 4.9 to 5.0 and minor peaks at pH 4.5 to 4.7 and 5.0 to 5.2; this resolution was greater than that achieved with rat liver angiotensinogens. Incubation of human angiotensinogens with neuraminidase for 3 or 16 h raised their isoelectric pH by about 0.5 U, probably due to removal of sialic acid. Since microheterogeneity persisted after desialylation, it is probably determined by structural characteristics other than sialic acid composition.