The Interpretability Hypothesis (Tsimpli et. al., 2003; Tsimpli & Dimitrakopoulou, 2007) claims that uninterpretable features which are not instantiated in L1 are unavailable for L2 acquisition. This hypothesis is supported (e.g. Hawkins & Hattori, 2006; Al-Thubaiti, 2011) and opposed (e.g. Rothman et. al., 2010; Bond et. al., 2011) by many other studies. Some of such studies were carried out on the L2 learners who were exposed to natural input in the target language by living in a country where it is spoken as a mother tongue; yet some others were carried out just on the ones who acquired it in their home country. In this respect, (not) being exposed to natural input in L2 acquisition might have played some role in the results obtained in such studies. The present study aimed to analyze the validity of the Interpretability Hypothesis and the possible role of positive evidence in L2 acquisition process. The data of the study were collected through a grammaticality judgment test, a wh-question formation task and a translation task. Along with a native control group (N:58), four learner groups were formed according to the place they live (USA or Turkey) and their level of proficiency in English (advanced or intermediate) (N:46, N:38, N:20, N:30 respectively). The results of the study stood against the Interpretability Hypothesis, emphasizing the importance of positive evidence in L2 acquisition process. According to the statistical test results, the uninterpretable (uwh*) feature appeared to be available only for the highly proficient L2 learner of English who are exposed to natural input in this language. As the findings of the study suggest, to assess the availability of UG in SLA precisely, such studies should be carried out on the participants who are exposed to natural input in the target language.