It may well be fungal contamination. Has the colour of the growth medium changed? Do you see any white cotton like growths in the medium? 

Usually, with bacterial contamination, colour of the medium changes greatly (eg from pink to yellow). Yeast contamination, you will see tiny black dots under the microscope. Fungal contamination, you will see a network of filaments under the microscope.

Just make sure to decontaminate your incubator with alcohol before putting any more cells in !

thanks Flourina.

i really can't say about the medium color because its turbid but i don't see cotton like growth.

i am wondering why not all cells contaminated why only few?

Definitely contamination, especially as it sounds like it is not in all wells. Best bet is to remove and throw away plate, disinfect everything and make fresh media and components. Not worth gambling with other cultures!

No doubt its a contamination, if you want to be sure, observe under high power( 40x or more) microscope and look for Brownian movement. You should be able to see it. 

By your description (only some wells are affected) it seems the contamination is not from your media but something else, just to be safe, keep your medium about 2 ml and any other liquid you used in that experiment overnight in the incubator to identify the source and observe for the growth in the morning., use standard laboratory cleaning procedures for hood, incubator, microscope area etc.

thanks all.

i used CD4T Cell Isolation Kit for non-human primate (#130-092-144),

which contains a Biotin-Antibody Cocktail