BACKGROUND It has been speculated that periodontal probes can transmit periodontal pathogens from site to site. The purpose of this study was to evaluate the potential for bacterial transmission as a function of periodontal probe design. METHODS Four different periodontal probes were used to measure probing depths ranging from 0 to 3 mm and > or = 4 mm. Following measurement of each pocket, probes were transported to a laboratory and cultured on blood agar, and colony forming units (CFUs) were determined for total aerobic, anaerobic, and dark-pigmented colonies (DPCs). Eight randomly selected probe tips, representing each probe type and probing depth category, were selected for examination by scanning electron microscopy (SEM) to determine distribution and morphotypes of adhering microbes. RESULTS Results showed no statistically significant main effect for probes with respect to CFUs. However, there was a statistically significant increase in CFUs for deeper pockets when compared to pockets 0 to 3 mm. SEM observations were consistent among groups, regardless of probe design or probing depth. All probes exhibited a rough surface texture. Microbes were observed as single organisms or in aggregates and were adherent along the entire length of the probe up to the maximum probing depth. CONCLUSIONS There was no difference in CFUs obtained as a function of periodontal probes. The rough surfaces of the probes used in this study likely promoted bacterial adherence through two different mechanisms: through scraping of the pocket walls by the roughened surfaces of the probe and by the high surface-free energy of the metal probes, facilitating bacterial adherence.