This study investigated three key issues in heritage language (HL) research. Previous research shows HL speakers have an advantage on oral production tasks compared to L2 speakers who instead perform better on written tasks. Furthermore, both L2 and HL speakers are claimed to have a "yes-bias" towards retaining ungrammaticality in GJTs. Finally, the morphological domain has been shown to be as problematic for HL as L2 speakers but research in lesser-known languages is needed. Adult L1, L2, and HL speakers of Italian were compared on an oral priming task and timed GJT. Accuracy and response times were elicited from the latter test. The forms investigated were object and "si"-passive pronouns which lack corresponding forms in Swedish, the dominant language of the bilingual groups. Mixed effect regression was modelled to accuracy on the priming and GJT and response time data from the GJT. In contrast, a "d-prime" analysis was used to measure the degree of sensitivity to grammaticality and bias towards correct and incorrect answers in the GJT. Overall, the two bilingual groups performed quite similarly across the measures tested. All three groups show high sensitivity to grammaticality and a very similar bias for yes-answers on both grammatical and ungrammatical items.