The abrasive effect of variuos porcelain surfaces on human enamel was studied in vitro. Sixty pairs of tooth-porcelain specimens were tested under a standard load and rate in distilled water and with and without intermitten exposure to a carbonated beverage. The amount of wear of enamel and porcelain specimens was determined after 5,000, 15,000, and 25,000 cycles. After 25,000 cycles there was no significant difference in the enamel wear betwen glazed and polished groups, but wear produced by the unglazed groups was significantly higher (P .05). Overall, exposure to a carbonated beverage significantly increased the amount of enamel wear produced by all porcelain surfaces (P .001). The finish of the porcelain surface did not influence its wear.