In studies of bilingual word recognition with masked priming, first language (L1) primes activate their second language (L2) translation equivalents in lexical decision tasks, but effects in the opposite direction are weaker (Wen & van Heuven, 2017). This study seeks to clarify the relative weight of stimulus-level (frequency) and individual-level (L2 proficiency, L2 exposure/use) factors in the emergence of asymmetrical priming effects. We offer the first data set where L2 proficiency and L1/L2 exposure/use are simultaneously investigated as continuous variables, along with word frequency. While we replicate the asymmetry in priming effects, our data provide useful insights into the factors driving L2–L1 priming. These fall almost exclusively under the category of stimulus-level factors, with L2 exposure/use being the only experiential variable to show considerable influence, although complex interactions involving L2 proficiency and word frequency are also present. We discuss the implications of these results for models of bilingual lexical processing and for the appropriate measurement of experiential factors in this type of research.