Ethylene is a small gaseous molecule that was discovered in the 19th century and was later recognized as the first plant hormone. It plays an important role in plant biology because it is involved in many aspects of the plant life cycle, including seed germination, root and flower development, fruit ripening, senescence, abscission and responses to biotic ( pathogen attack) and abiotic (wounding, hypoxia, chilling, floods and droughts) stresses. Ethylene is synthesized from 1-aminocyclopropane-1-carboxylic acid (ACC) where the reaction is catalyzed by the enzyme ACC oxidase. ACC oxidase is expressed in all vegetative and reproductive plant tissues. The aim of the study was to find out the location of ACC oxidase in the developing fruit of tomato. To localize the enzyme, we used immunolocalization with specific primary antibodies against ACC oxidase and examined the effect of different buffers in antigen retrieval. Using immunolocalization we detected ACC oxidase specifically inside the fruit and in the vascular region of the fruit and fruit pedicel. In the step of antigen retrieval during the immunolocalization process we used three different buffers that resulted in differences in the intensity of specific staining and the amount of nonspecific background staining. The strongest specific staining with the lowest background was achieved by using citrate buffer pH 6 in the step of antigen retrieval. Negative controls were stained using the nonimmune serum instead of specific primary antibodies. No specific staining was observed in the control sections.