We have developed a method for detecting sister chromatid exchanges using premature chromosome condensation and immunogold labelling followed by silver enhancement. Cells were grown in bromodeoxyuridine before sister chromatid exchanges were generated with mitomycin C. Calyculin A was then used to condense the chromosomes prematurely. Finally, incorporated bromodeoxyuridine was detected using a gold-conjugated antibody followed by silver enhancement. Exchanged chromatids could easily be seen in the resulting chromosomes. This simple method allows sister chromatid exchanges to be detected with great sensitivity and resolution.