PLATES XLVII-XLIX

SEVERAL species of normally saprophytic phycomycetes are recognised as opportunist pathogens of man and domestic animals (Ainsworth and Austwick, 1959; Baker, 1971). In most instances the fungal infection is secondary to a recognisable predisposing condition. However, in mycotic abortion of cattle, in which various mucoraceous and other types of fungi have been implicated (Ainsworth and Austwick, 1959), no predisposing factor has been identified although placental invasion may be promoted by fungal growth-stimulants present in the foetal cotyledon tissue (Corbel and Eades, 1973; Eades and Corbel, 1973). In many cases Absidia ramosa (A. corymbifera) is the only organism implicated in the infection. A. ramosa and other phycomycetes also occasionally produce extra-placental mycotic lesions in young or adult animals of various species. As in the case of bovine mycotic abortion, these infections are sporadic with the majority of animals exposed to natural infection having a high degree of resistance. The nature of this resistance and the reasons for its apparent failure in those animals developing active infection are as yet undetermined. In the present study an attempt has been made to characterise some of the factors that determine susceptibility and resistance to phycomycotic infection. Experimentally produced A. ramosa infection in mice has been used as a model for examining these. This test system was selected because mice are susceptible to both natural and experimental infection with A. ramosa (Symeonidis an