Culture of HT 29 Colon cancer cells

[Warnakulasuriya Lankaputhra]

I found a contamination which can not be killed by Penn-strep or Gentamycin 
in my HT 29 colon cancer cell culture.  I think it could be a protozoan 
contaminant, rather than bacterial.

Could some one tell me how to get rid of non-bacterial contaminants.  Has 
anyone experienced such 'hard-to-kill' contaminants in these cell lines.

Thanks,

Lanka.