Two experiments were conducted to investigate knowledge transfer from learned programming languages to learning new ones. The first experiment concerned transfer from knowing LISP to learning PROLOG; the results showed that subjects who knew LISP had significant advantages over subjects who did not. Moreover, among the subjects who knew LISP those who knew LISP better seemed to learn PROLOG faster. The second experiment studied transfer from knowing either PASCAL or PROLOG to learning LISP; attention was specifically focused on transfer of knowledge of writing recursive and iterative programs in these languages. The results indicated that PROLOG programmers, who were usually more knowledgeable on recursion, were more ready to learn the recursive part of the LISP language. Some general theoretical discussion about knowledge transfer among programming languages is also presented in the paper.