Examines lexical interference from the perspective of language proficiency in a study of Iranian English-as-a-Foreign-Language learners. Subjects were given two tests--a proficiency test and a test on similar lexical forms--to investigate whether there is any relationship between the two variables in question. Results indicate that there is a strong negative correlation between learners' language proficiency level and the number of errors they make due to interference. (Author/VWL)