Objective: This study evaluated the biocompatibility of a one-step dentin bonding agent (Prime&Bond 2.1) in pulp capping compared with calcium hydroxide [Ca(OH)2]. Method and materials: Thirty sheep teeth and 20 intact human premolars were used. After cavity preparation, pulp exposure was achieved with a bur (#390). Adhesive pulp capping was performed in 25 teeth (15 sheep and 10 human). In the control group (12 sheep and 10 human teeth), pulps were capped with Ca(OH)2 and all of the cavities in both groups were sealed with resin composite. Three of the sheep teeth were used as intact controls. Teeth were extracted 7 or 90 days following treatment and prepared for histological examination and bacterial detection. Results: At 7 days, severe inflammatory responses underlying the bonding agent and in the coronal pulp were observed with soft tissue disorganization in both human and sheep teeth capped with Prime&Bond 2.1. All of the teeth capped with Ca(OH)2 exhibited mild inflammatory reactions limited with the perforation area. After 90 days with the bonding agent, in 3 of 9 sheep teeth, chronic inflammatory reactions were significant, while slight pulpal reactions were observed in the others and dentin bridge formation in all of the sheep teeth was found. However, in human pulps, persistent, unresolved inflammation with the lack of dentin bridge formation was observed. In the Ca(OH)2 group, pulp repair with dentin bridging was found in all of the teeth, both sheep and human. No correlation was found between the presence of inflammation and bacterial staining using Spearman rank correlation test (P > .05). Conclusion: Prime&Bond 2.1 facilitates enhanced pulp healing and bridge formation in sheep teeth, but in human teeth it was not as successful as Ca(OH)2 as a pulp capping agent. Keywords: biocompatibility, dentin bonding agents, pulpal response