filter a lookup, based on another lookup

filter a lookup, based on another lookup

I have a small database holding info on the following
candidates
employers
vacancies
referrals
 
Vacancies are added with a lookup to the employers
 
now in the referrals table I want to pick a candidate, then pick an employer and have another drop down listing the vacancies for that employer instead of all vacancies.
 
can anyone point me in the right direction ?
 
cheers
 
Mitch