Virus diseases are an important factor worldwide in crop production losses. Correct diagnosis of a disease is essential and in developing countries in the absence of modern facilities virologists regard infectivity tests as an important tool in identifying viruses. However compared with progress in other areas of plant virology infectivity assays lag behind. These tests should be carried out under optimum environmental conditions to permit symptom development. Environmental conditions should be standardised. Results of infectivity tests should be interpreted in relation to variation in the environment and the use of a known cultivar and its source. The use of an indicator host relates indirectly with variation in the environment and consequently symptom development. A suitable host must be used if the virus is to initiate infection. Plants used as indicator hosts must be genetically identical and from the same origin. Sensitive hosts are required to index seed batches in a well organised seed certification programme. There is a need for international seed and serum centres in strategic regions among developing countries and close collaboration between virologists to list genetically identical test plants for infectivity assays under their own conditions. Other factors to improve identification include an understanding of insect vectors. When used properly these tests can play a significant role in identification of virus strains and screening for resistance against virus diseases of economically important cash and food crops.