The morphologic changes occurring in human chromosomes during G-banding by trypsin treatment on the same metaphase were followed with the aid of an atomic force microscope (AFM). It was found that trypsin treatment alone caused a pattern of collapse in the chromosomes that was clearly dependent on the duration of trypsinization. The progressive pattern of collapse first indicated the loss of internal differentiation between chromatids, then bands, and finally all internal structures, except for edges running around the chromosomes' perimeter. When stained with Giemsa, the collapsed chromosomes partly regained their original form, and transverse ridges appeared that correspond to G-positive band regions. However, the treatment of fixed chromosomes with trypsin for 42 s diminished the chromosomal edges, and the z-dimensions could not be measured even with the subsequent application of Giemsa.