Strains of Saccharomyces cerevisiae expressing P28-I, an antigen inducing protection against schistosomiasis, have been constructed. Transformants containing a very high copy number of a P28-I expression vector were selected by genetic complementation involving deficient LEU2 or URA3 alleles carried by plasmids. Using the ura3 fur1 auto-selection system, constitutive and stable expression of P28-I could be obtained in cultures grown in rich medium. The accumulation of the foreign protein exceeds 25% of total yeast proteins when estimated by Coomassie Brilliant Blue staining of SDS-PAGE. Moreover, P28-I which was located intracellularly was soluble and biologically active.