Change of first language (L1) status from the most stable language to a less accessible language over the life-span of a bilingual individual is termed ‘language attrition’. Such a shift in ease of L1 access has been reported to affect the lexicon (Pelc, 2001) more than other aspects of language. However, whether L1 attrition is affected by reduced L1-strength or increased second language (L2) interference is unresolved. This study was designed to understand the relative contributions of L1-strength (Ebbinghaus, 1885; Paradis, 2001, 2007) and L2-interference (Loftus & Loftus, 1980; Gürel, 2004) towards L1 attrition in L2-dominant bilingual individuals, and how attrition is affected by language use and proficiency. We used a cross-modal (picture-auditory word) and cross-linguistic (Bengali-English) lexical priming paradigm in order to test the nature of L1-L2 interaction in 27 Bengali-English-speaking individuals. Participants were divided into two groups (L1-dominant and L2-dominant) varying in relative L1-L2-proficiency. Familiarity ratings for English words and their translation equivalents permitted generating four word-pair categories: HighEnglish-HighBengali, LowEnglish-LowBengali, HighEnglish-LowBengali and LowEnglish-HighBengali. Reaction time (RT) and Event related potentials (ERPs) were recorded to a syllable-judgment task for the auditory word. Participants also rated themselves on language and reported language use. We hypothesized that if reduced L1-strength affected L1 attrition, Bengali words from all four word-pair categories would elicit longer RTs and larger negative ERPs than English words. In contrast, if L2-interference affected L1 attrition, all Bengali words except ones from the LowEnglish-HighBengali category would elicit longer RTs and larger negative ERPs compared to their English translations. Results showed participants’ L1-use and L2 self-ratings predicted performance in L1. Behavioral data showed longer RTs for Bengali than English in the HighBengali-HighEnglish category and longest RTs for English in the LowEnglish-HighBengali category. ERP data showed greater negativities to English and Bengali words from the category with low familiarity English ratings regardless of their Bengali ratings. Different results from the two measures of the lexical task suggest that each task reflects a different point in the process of lexical access. Overall, findings suggested that L2-interference into L1 plays a larger role in L1 attrition in L2-dominant individuals.