Active and passive sentences were presented with probable and improbable semantic content to 100 first graders and 100 kindergartners. "Irreversible" sentences were considered improbable. In a design employing syntax, probability, grade, and sex as factors, probability and syntax were found significant both as main effects and in their interaction. Probability had little effect on the comprehension of active sentences, but strongly affected comprehension of passive sentences. First graders responded correctly more often than kindergartners; the difference was greatest on improbable sentences, with improbable passive sentences the most difficult. Sex differences were not found. The greater difficulty in comprehending less familiar sentences when syntactic form is not supported by semantic content suggests that the semantic component of grammar may play an important role in the child's acquisition of syntactic comprehension. (Author)