Monoclonal antibody Am1 against conservative epitope of tryptophanyl-tRNA synthetase (WRS) was labeled with colloidal gold particles and used to localize the enzyme on ultrathin sections of eubacteria (Escherichia coli), archaebacteria (Methanococcus halophilus), rat pancreas tissue and rat fibroblasts (cell line RAT1). In all cell types immunoelectron microscopy revealed predominant cytoplasmic location of gold particles, as this could be expected from known biochemical data. In particular, in mammalian cells intensive labeling was observed in cytoplasmic regions rich in polysomes and free ribosomes. At the same time, the label was virtually absent in cytoplasmic regions where microfilament bundles were present. Significant concentrations of gold particles were found in mitochondria and nuclei. In the latter case, gold particles were located over diffuse chromatin regions and were virtually absent over compact chromatin. The density of diffuse chromatin in labeling may amount to about 50% of that found in the cytoplasm. Distribution of labeled antibodies over E. coli cells looks rather similar to that found for M. halophilus: gold particles are preferably concentrated over the cytoplasm and "boundary zone", i.e., a 30 nm wide cytoplasmic zone adjacent to the nucleoid border, while the label over nucleoid is virtually absent. Two main conclusions are drawn: (i) although in the animal cell homogenates WRS is recovered mainly as a soluble cytosolic enzyme, in intact cells it is associated with defined cellular organelles and compartments; this may be an evolutionarily acquired feature probably typical for multicellular organisms; (ii) the considerable density of labeling in diffuse (not compact) chromatin regions may be indicative of WRS involvement in the active chromatin functions (transcription, processing, transfer of gene products, etc.).