Contamination and its fungus - persistant fungal contamination in cell cultures (Dec/10/2008 )

I have sp2/o cell lines often brought to my lab from other labs. First they are perfectly fine. But within a few days i find it contaminated with fungus. I have gone ahead and cleaned the whole lab i.e. laminar air floor. CO2 incubator, every instrument there. checked my media for its sterility. Yet i find fungal contamination. Please help me as i am to start growing hybridomas.

Stocks contaminated? It certainly doesn't sound like an adventitious fungal infection.