CEA and NCA show an increased mobility in SDS-polyacrylamide gel electrophoresis if the samples are preheated in 1% SDS for 5 min at 100 degrees C. Now we found that the slow-to-fast form conversion of CEA and NCA also occurs under milder conditions and is reversible at low SDS concentration. At 100 degrees C the conversion was completed within few minutes in 0.03% SDS, while at lower temperatures it required higher SDS concentration and longer time of incubation. After the antigens were heated at 100 degrees C at 0.05% or lower SDS concentration, they showed slow reassociation at room temperature. Under conditions insufficient for the complete slow-to-fast or fast-to-slow form conversion two CEA or NCA bands were present at various proportions. No material with an intermediate mobility was detected for CEA, while a weak transient intermediate band (located closer to the fast band) was visible in some gels showing an incomplete dissociation or association of NCA. The ratio of mol. wts. of slow and fast band was approx. 2:1 for NCA and 1.5:1 for CEA. Despite some deviations from the expected mol. wt. of the presumably undissociated CEA, the results obtained supplied a support for the conclusion that the two electrophoretic forms of CEA and NCA represent the dimer-monomer interconversion.