This paper addresses perceived blocking effects of the English adjectival prefix un- and the accuracy of its identification as a negator. Since un- appears to pattern among adjectives according to antonymy, I posit that the behavior of un- can be fully explained by admitting the value {ANTONYM} as a prime semantic component in lexical composition, and by identifying un- with this value. We find that {ANTONYM} seems to belong to certain intersective adjectives, to all privative adjectives, and to no subsective adjectives. The addition of un- is restricted to those adjectives which neither contain the component {ANTONYM} nor exist in a subsective pair. With this analysis, I hope not only to comprehensively explain the distribution of un-, but also to provide evidence that there exists a prime semantic component {ANTONYM}.