A method for identifying whether an agent is capable of blocking the formation of a mammalian-infective form of a parasitic protozoan which comprises in vitro subjecting a non-infective form of the parasitic protozoan in the presence of different amounts of the agent to be identified to a temperature of about 37.degree. C. for a sufficient time to cause the parasitic protozoan to differentiate to the mammalian-infective form, recovering total mRNA from the resulting differentiated parasitic protozoan and detecting in the mRNA so recovered a mRNA transcript encoding a heat shock protein associated with the mammalian-infective form of the parasitic protozoan, which transcript is present only in the mammalian-infective form of the parasitic protozoan, wherein the detection involves contacting the total mRNA recovered from the parasitic protozoan under hybridizing conditions with a detectable oligonucleotide which is complementary to at least a portion of the mRNA transcript which encodes the heat shock protein associated with the mammalian-infective form of the parasitic protozoan and detecting the formation of hybridization complexes between the mRNA transcript and the detectable oligonucleotide and thereby the presence of the mRNA transcript encoding the heat shock protein associated with the mammalian-infective form of the parasitic protozoan, the absence of such mRNA transcript indicating that the agent has blocked the formation of the mammalian-infective form of the parasitic protozoan so as to thereby identify the agent as being capable of blocking the formation of the mammalian-infective form of the parasitic protozoan.