One of the main aims of the Teaching Content in a Foreign Language (TCFL) program, an in-service teacher training program at the Continuing Education Centre of the University of Jyvaskyla (Finland), was to improve the participants' proficiency in English. To promote this objective, English was made the operating language for the whole program. English was used as the language of instruction and discussion, almost all material was in English, and the participants were encouraged to use English in their informal contacts as well. The principle of "English only" worked well as there was an extensive exposure to spoken and written English. Participants' proficiency was tested at the beginning and end of the program. The proficiency test was based on a nine-point proficiency scale and consisted of listening comprehension, speaking, reading, writing, vocabulary and grammar sections. A sample diploma of proficiency in teaching in English is appended as well as the overall proficiency rating scale. (CK)