Constitutive heterochromatin in mammalian chromosomes was first demonstrated in 1970 by hybridisation of radio-labelled DNA with the DNA of cytological preparations. The technique has been modified subsequently to reveal either ‘C’ or ‘G’ bands; the latter may be produced by a variety of techniques utilising proteolytic enzymes, urea, detergents, and alkaline phosphate-buffered solutions. Thus far, however, ‘C’ banding techniques do not satisfactorily identify ‘G’ bands, and vice versa.We have developed a technique utilising sequential 0.1 M NaOH and 0.05% trypsin treatments which consistently reveals both ‘C’ and ‘G’ bands in the same preparation. Furthermore, this method shows a clearer delineation of the ‘C’ bands than is usually seen by the standard ‘C’ band methods. Thus, we have observed (1) consistent differentiation of the ‘C’ band of the #1 chromosome into three distinct bands (2) frequent similar differentiation of the ‘C’ band of the #9 chromosome, (3) differentiation of the Yql2 band into at least two bands.Investigation of a patient with 46,XX, inv(12)(p12q11) by this technique indicates that the functional cetromere does not occupy the whole of the #12 ‘C’ band. A similar phenomenon probably accounts for the ‘inactivated centromere’ observed with some translocations.