Based on the assumptions that a universal grammar has a number of functional parameters and that in each language, some are not activated, a study was undertaken to investigate two hypotheses. They are (1) that in a grammatical situation where an adult's first language parameter is not activated in the second language, the learner will "lose" the first language parameter, carrying over some of its structures into the second language; and (2) that in losing the first language parameter, all of its aspects will be lost together, not partially. The parameter examined was the "pro-drop" parameter, which includes such properties as the ability to omit subjects, and free inversion of subject and verb in declarative sentences. The subjects were 54 native speakers of Spanish (a pro-drop language) learning English (not a pro-drop language) and a control group of English students who were native speakers of French (like English, not a pro-drop language). Results suggested that having to change a parameter in language learning may cause problems and is a source of interference errors, at least initially. Other possible explanations for the results are discussed. (MSE)