The effect of translation knowledge on bilingual lexical production is mixed, with some studies showing translation interference and others showing facilitation. We considered the roles of first-language (L1) translation knowledge and second-language (L2) proficiency in lexical retrieval, testing predictions of the competition for selection, frequency lag and activation boosting accounts. In experiment 1, 54 highly proficient Spanish–English bilinguals named pictures of low-frequency nouns in English (L2). Spanish (L1) translation knowledge and English proficiency had an interactive effect on tip-of-the-tongue experiences with increased L1 translation interference at low levels of L2 proficiency and facilitation at high levels of L2 proficiency, consistent with combined predictions of competition for selection and activation boosting accounts. Experiment 2 confirmed that confounding lexical variables did not drive translation effects. By examining individual differences within bilinguals, we found support for multiple mechanisms that play a role in bilingual lexical retrieval that were not evident at the group level.