This paper examines semantic constraints governing the occurrence of interjections with various other types of grammatical phenomena. Four interjections, "oh,""ah,""say," and "well," which typically occur embedded in sentences, are discussed in terms of their semantic properties and possible contexts. It is concluded that: interjections do not occur inside sentences unless there is a plausible semantic alternative to the element which follows them and which they refer to; "oh" and "well" can be used if the speaker is simply trying to think of alternatives, but that "say" and "ah" have more stringent conditions attached; and that there are numerous grammatical phenomena with which any or all of these interjections are incompatible. These phenomena include: certain types of pronouns, idioms, final sentential adverbs, negative polarity items, presupposition-suspending "if" clauses, picture noun clauses, preposed adverbials, and topicalized words. It is further concluded that the behavior of interjections can reflect or reveal unusual and poorly understood properties of phenomena such as picture noun clauses and preposed adverbials. (Author/AM)