In epidemiological studies of the apricot vascular pathogen Eutypa armeniacae Hansf. & Carter in this laboratory (Moller and Carter 1965;Carter and Moller 1967) some diagnostic problems have been encountered. The pathogen is sufficiently variable in culture to be confused with other Ascomycetes, such as species of Valsa and Cryptovalsa which may colonize dead vascular tissue below pruning wounds which have been invaded by E. armeniacae. It is also difficult to differentiate E. armeniacae ascospores from those of related Ascomycetes caught in spore traps during wet weather.The preparation of antisera to antigens of E. armeniacae ascospores and mycelium and their possible use in identification of the fungus are the subjects of this communication. Materials and MethodsPerithecial stromata of E. armeniacae were soaked in water for I hr and attached to the lid of a closed container for several hours to collect the liberated ascospores. The ascospores were then suspended in a small quantity of distilled water and the concentration estimated by haemacytometer. A quantity of the suspension containing 2-5 X 107 ascospores was then emulsified with an equal volume of Freund's complete adjuvant and injected subcutaneously into several sites on the flank of a rabbit. Two further injections containing similar quantities of spores were administered 1 and 5 weeks after the initial injection. The rabbit was bled through an ear vein 1 week after the final injection and several more times during the month following.E. armeniacae was cultured on Czapek-Dox agar and after 2-3 weeks of growth the hyphae were scraped off the agar, suspended in 0 ·14M N aCI, and ground in a glass tissuehomogenizer. Microscopic examination of the suspension showed the presence of small pieces of hyphae. This material was emulsified with an equal volume of adjuvant and used to immunize a rabbit as described above for the ascospore antigen.Serological tests with mycelial antigens were carried out by the double-diffusion precipitation technique (Crowle 1961) using 0·75% agar in 0·02M phosphate buffer, pH 7,2, containing 0'14M NaCl and 0·02% sodium azide. Agglutination tests using ascospores as antigens were carried out on microscope slides. Immunofluorescence tests with both mycelium and ascospores were carried out by the indirect method (Weller and Coons 1954). Smears of the antigens were prepared by drying suspensions on microscope slides which were then irrigated with rabbit antiserum for 15 min. The smears were washed thoroughly with 0 ·14M NaCl and irrigated with fluorescein isocyanate-labelled sheep anti-rabbit globulin serum for 15 min and again washed with 0 ·14M NaCl. The smears were mounted in glycerol and examined in a Leitz Ortholux microscope equipped with both phase-contrast and ultraviolet illumination.* Manuscript