When to use "-san"

For the last four years I have provided technical advice to a department of the Japanese Government. Prior to the beginning of this year, they always addressed me as "Dr X" or "Mr X.Y" and I always addressed them as "Mr/Ms X". Now they have started to address me as "X-san"; Should I reciprocate?

Cris,

I suggest using English titles when speaking English, and Japanese honorifics when speaking Japanese. Mixing them sounds silly and is considered patronizing by many people.

Are you conversing with Japanese people? IF so, you should be using 'san' all the time. I find it patronising to call Gaijin 'san' though.

Actually, a lot of government officials are supposed to be called "-sensei" or with other titles like "-kachou" etc. Dr. Whetton, if there is a reliable interpreter who deals with both you and the officials, you should ask that person. Otherwise, stick with the "Mr." which would be safest.