Immunoelectron microscopy using ultrasmall gold markers is a very sensitive method to detect molecules at high resolution. In order to discriminate the gold particles in the electron microscope, enlargement of gold particles is necessary. So far, mostly silver ions were used for deposition onto the surface of gold grains. In our study, we tested the selective deposition of gold instead of silver ions to enlarge gold particles. This was performed following immunogold detection of DNA at the surface of ultrathin sections embedded in the acrylic resin LR White (postembedding approach). Morphometric analysis of the distribution of DNA in human spermatocytes revealed that the method offers very good specificity and sensitivity and therefore is a good alternative to the use of silver for signal enhancement. This technique was also applied to the detection of ribosomal genes in human testis at the electron microscopic level by in situ hybridization. Ribosomal genes were localized in peri- and intranucleolar chromatin as well as in the dense fibrillar component of nucleoli.