Blue and white

[FABIAN MOEBIUS]

Hello,
It takes my blue/white screen more than 48 hours to become blue (pBS 
in XL-1 Blue), but after a long while the colonies  become deep blue. We spread 100  l of 100 mM IPTG and 40  l of Bluo-Gal 
(Sigma, 40 mg/ml) per 10 cm dish. Which factors influence color 
development? Is it critical to dissolve X-gal in dimethylformamide?
Thanks

Dr. Fabian F. MOEBIUS
Institut fuer Biochemische Pharmakologie
Peter Mayr Str. 1
6020 Innsbruck
AUSTRIA
TEL ++43-512-507-3155
FAX ++43-512-588627