A method is proposed for estimating the sizes of polymorphic C-bands of human chromosomes by microdensitometric measurement of the DNA content of whole chromosomes. The method requires measurements of: a chromosome known not to be polymorphic, as a standard against which to normalize all measurements; the polymorphic chromosomes of interest; and a homologous polymorphic chromosome lacking the C-band. In an example studied here, the C-band of chromosome 9 was estimated to contain about 0.0296 pg of DNA.