SUMMARYA method for reflectancein situhybridization (RISH) is presented. The importance of the method is demonstrated by results obtained on cytological and histological breast cancer specimens.Scattering reflectance signals from 1‐nm colloidal‐gold particles after RNA/RNAin situhybridization, using digoxigenin‐labelled riboprobes, were detected by confocal scanning laser microscopy.The mRNA expression of two ras‐related genes, rho B and rho C, was analysed in human histological breast cancer specimens and in human breast cancer cell lines. Horizontal (x, y) and vertical (z) optical sections after three‐dimensional imaging were used for visualization.A marked heterogeneity (between individual cells and between specimens) was noted for the expression of the rho B gene, both in cytological and in histological samples. On the other hand, rho C was always expressed and showed no heterogeneity.This method allows the identification of several cellular constituents in an heterogeneous tissue structure, as demonstrated by the simultaneous detection of rho B (or rho C) by reflectance and of DNA, cytokeratin and/or vimentin by flu