Mycoplasma contaminants of animal and human cell cultures were rapidly detected and identified by an indirect immunofluorescent technique. Cells suspected of being contaminated by mycoplasmas were grown as monolayers on chamber slides in a culture medium selected to promote mycoplasmal growth. Before fixation by acetone, the monolayers were subjected to a hypotonic treatment to cause swelling of the mycoplasmas. Detection and identification were then performed by indirect immunofluorescence using rabbit antisera to various mycoplasma species. The correlation between results obtained by the standard isolation procedure and those obtained by this method was very close.