OBJECTIVE Immunologic recognition of pregnancy is important for normal gestation. Successful pregnancy is characterized by a Th2 dominance, whereas there is a Th1 dominance in failed pregnancies. We assume that a signal given by the fertilized ovum induces a Th2 shift, necessary for a normal outcome. In vitro fertilization provides a tool for testing this possibility. METHODS Phytohemagglutinin-stimulated lymphocytes were incubated for 48 hr in the presence of culture media from in vitro fertilized eggs, as well as in follicular fluid (FF) and control supernatants. Total RNA was isolated from the lymphocytes by the guanidine-isothiocyanate method and interleukin (IL)-10 mRNA expression was detected by reverse transcription-polymerase chain reaction. RESULTS Ten percent of the activated lymphocytes incubated with FF expressed IL-10 mRNA, whereas 88% of the lymphocytes activated with supernatants of sperm + oocytes gave a positive signal. Significantly (P < 0.05) fewer (50%) lymphocytes stimulated in the presence of control supernatants also expressed mRNA for IL-10. In supernatants of activated lymphocytes incubated with the culture medium of spermia + oocytes, the concentration of IL-10 was significantly higher than in the lymphocytes incubated with FF. CONCLUSIONS These data suggest that the presence of the fertilized ovum induces a Th2 shift, which enables pregnancy to proceed to term.