Prior knowledge of the content of a passage should reduce the effort required to encode the passage, and facilitate its recall. The results of two experiments on the effects of prior knowledge upon comprehension of simple technical prose are presented. The procedure was to collect ratings of the amount of prior knowledge for individual passage sentences, and then determine whether those familiarity ratings predict reading (study) time and recall for the sentences. Results are patterned on the idea that increased study time is required for unfamiliar material due to the additional processing required to encode an unknown proposition and to elaborate less familiar propositions. The results are discussed in terms of possible theoretical mechanisms for the use of prior knowledge in prose memory. (Author/PN)