There are difficulties in the diagnosis of deep-seated candidosis; the clinical features are non-specific and similar to bacterial infections, and blood cultures may be negative even when there is an active invasion of the deep tissues. Serology, especially precipitin tests, are considered a useful diagnostic adjunct (Taschdjian, Seelig and Kozinn, 1973; Odds, 1979) but precipitins to extracts of Candida albicans have been detected in sera from apparently normal subjects (Douchet and Müller, 1972; Everall, Morris and Morris, 1974), pregnant women (Stanley, Hurley and Carroll, 1972), burned patients (Oblack, Schwartz and Holder, 1976), patients given intravenous hyperalimentation (Glew et al., 1975) and in patients after open heart surgery (Murray, Buckley and Turner, 1969; Parsons and Nassau, 1974; Evans and Forster, 1976). These 'false positive' antibodies may often be directed to the cell-wall mannans of C. albicans (Pepys et al., 1968). Although attempts have been made to introduce new tests for candida antibodies (Stickle et al., 1972; Müller, 1974) it is unlikely that any serological test can be more specific than the antigenic reagents on which it is based. Most efforts to overcome the problem of poor specificity have involved either the modification of candida extracts by removal of the mannan antigens (Long-bottom et al., 1976; Syverson and Buckley, 1977; Syverson, Buckley and Gibian, 1978) or a search for novel antigenic components unique to the hyphal form of the fungus (Evans et al., 1973; Syverson, Buckley and Campbell, 1975). Staib and his co-workers (1977a and b) have suggested that an inducible acid proteinase secreted by C. albicans may be a useful diagnostic antigen; but they appear to have tested only unrefined culture filtrates containing the enzyme, with equivocal results. This paper describes a simple method for the rapid purification of C. albicans proteinase which is a development of the procedure of Remold, Fasold and Staib (1968). The value of the purified enzyme as an antigen for use in the diagnosis of systemic candidosis is assessed in tests with sera from experimental animals and humans.