The ability of a microorganism to adhere to a solid support and to initiate a colony is often the first stage of microbial infections. To date, studies on S. cerevisiae cell-cell and cell-solid support interactions concerned only cell agglutination during mating and flocculation. Colony formation has not been studied before probably because this species is not pathogenic. However, S. cerevisiae can be a convenient model to study this process, thanks to well-developed genetics and the full knowledge of its nucleotide sequence. A preliminary characterization of the recently cloned essential IRR1 gene indicated that it may participate in cell-cell/substrate interactions. Here we show that lowering the level of expression of IRR1 (after fusion with a regulatory catalase A gene promoter) affects colony formation and disturbs zygote formation and spore germination. All these processes involve cell-cell or cell-solid support contacts. The IRR1 protein is localized in the cytosol as verified by immunofluorescence microscopy, and confirmed by cell fractionation and Western blotting. This indicates that Irr1p is not directly involved in the cell-solid support adhesion, but may be an element of a communication pathway between the cell and its surroundings.