Seventeen different versions of a film designed to teach the assembly of the breech block of the 40mm antiaircraft gun were tested on 2377 apprentice seamen in 1948. Each version was shown to at least 100 men, in groups of about 30, in an effort to evaluate the relative effects on learning of certain content and tempo characteristic s of specific instructional films. The film versions differed with respect to six variables, designated as Verbalization, Rate of Development, Nomenclature, Errors, 'How it works', and Repetition. None of the men had ever assembled the breech block or similar mechanisms. They were grouped according to age, length of education, General Classification and Mechanical Aptitude Test Scores. Immediately after the film showing, each man was required to attempt to assemble the parts of the dismantled breech block. Ten trials were given, and a time score obtained for each man. The most significant results were that a slow rate of development, the pointing out of errors to be avoided, and repetition of the assembly demonstration, markedly and positively contributed to the effectiveness of the film. (GO)