1. Immediately after arrival the culture as well as the agar growth medium is macroscopically inspected for colonies of bacteria or yeasts, for the growth of fungi or for mites. Inspection for mites is usually carried out under a stereo magnifier (magnification 8x - 20x) If one type of these contaminations can be detected already by these means the depositor is informed and asked for a new sample.

 2. A small amount of callus or suspension cells is transferred by an appropriate sterile instrument (callus = forceps, suspension = pasteur pipette) onto a microscope slide. The cells are diluted in a droplet of water and the sample is covered with a cover slip. The sample is carefully inspected under the microscope (magnification 400x, phase contrast).

 3. Specific attention has to be paid to any moving particles in the sample. It has to be taken into account that contaminating bacteria will have about the same size as subcellular microsomal structures, plastids or mitochondria.

 4. If a contamination can be detected without any doubt the cultures are discarded and the depositor is informed and asked for a new sample (PC/1998/2/3 Appendix 1).