An atomic force microscope (AFM) has been used to quantify directly the adhesion of metabolically active Saccharomyces cerevisiae cells at a hydrophilic mica surface, a mica surface with a hydrophobic coating, and a protein-coated mica surface in an aqueous environment. The measurements used "cell probes" constructed by immobilizing a single cell at the apex of a tipless AFM cantilever. Adhesion was quantified from force-distance data for the retraction of the cell from the surface. The data indicated stretching and sequential bond-breaking as the cell probe was retracted from all of the surfaces. Detailed studies were made for physiologically active cells, which were shown to have different adhesion properties to glutaraldehyde-treated cells. Greatest cell adhesion was measured at the hydrophobic surface. Prior adsorption of a bovine serum albumin protein layer at the hydrophilic surface did not significantly affect cell adhesion. Changes in yeast surface hydrophobicity and zeta-potential with yeast cell age were correlated with differences in adhesion. Cells from the stationary phase adhered most strongly to a mica surface. Time of surface contact was demonstrated to be important. Both the force needed to detach a cell from a hydrophilic mica surface and the length of the adhesive interaction increased after 5 min contact. The AFM cell probe technique gives unique insights into primary colonization events in biofilm formation. It will continue to aid both fundamental studies and the assessment of new procedures that are designed to lower cell adhesion at surfaces relevant to biotechnology, medicine, and dentistry.