Quantitative expression of C gamma-allotype H4 of mink immunoglobulins was studied by enzyme-linked immunosorbent assay. The results presented suggest that production of H4 is under specific regulation. the concentration of H4 varies three orders of magnitude (10-10,000 micrograms/ml) from one mink to another. Fifteen percent of the sera of normal minks have the low H4 concentration, undetectable by the standard procedure of double immunodiffusion routinely used to test mink IgG allotypes. However, expression of these 'minor' allotypes may be significantly enhanced by hyperimmunization. Instability of this kind seems to be the main cause of earlier described deviations from Mendelian inheritance of C gamma-allotypes H2, H3 and H4.