Method of fixing fragile organisms (such as the PPLO group) by touching the top of an excised colony in agar on to a coverslip, then: (a) allowing a liquid fixative such as Bouin’s to percolate through agar and colony on to the coverslip; or (b) removing agar and colony and flaming the coverslip gently. Such preparations are stained by Gram or dilute Giemsa.