Sera from 35 apparently normal humans, 37 compromised human patients, 30 hedgehogs and 30 sheep, were examined for precipitating antibodies to four opportunistic fungi - Absidia corymbifera, Aspergillus fumigatus, Candida albicans and Rhizopus arrhizus-using counterimmunoelectrophoresis (CIE). Precipitins to A. fumigatus were almost exclusively confined to specimens obtained from the compromised human group (51% of those examined) while Candida precipitating antibodies were detected in the sera of both normal (26%) and compromised (49%) humans and in 10% of the hedgehog specimens. Serum precipitins against the two phycomycetes included in the investigations were rare. Because of the complexity of most fungal antigen extracts, it appears the two phycomycetes included in the investigations were rare. Beause of the complexity of most fungal antigen extracts, it appears essential that sera be tested against a number of different antigen concentrations if CIE is to be used with confidence in fungal serology.