The present work was aimed to assess the possible effect of stimulation of the maternal immune system on the teratogenic potential of diabetes mellitus. ICR female mice were immunized with splenocytes of male rats 3 weeks before the beginning of mating and were injected with 240 mg/kg streptozocin (STZ) 10 days after immunization. Females with blood glucose levels over 27.8 mmol/l and HbA_{1 c} levels over 6 standard deviations (SD) above the mean of intact animals were used for teratological studies. The rate of malformed fetuses, resorptions and fetal weights were evaluated for animals killed on day 19 of pregnancy using routine teratological methods. Also, phenotyping of spleen cells of these females was performed by fluorescein activated cell sorter analysis. Two main effects possibly due to immunostimulation of ICR females were observed: 1) immunostimulated females had significantly fewer litters with malformed fetuses than non-immunized females: only 4 litters out of 22 (18 %) compared to 10 out of 16 (63 %). Correspondingly, the incidence of malformed fetuses was also decreased: 2.1 compared to 8.9 %; 2) a significant increase in the pregnancy rate in immunized diabetic ICR mice: 69 % as compared to 44 % in non-immunized diabetic females. Also, immunostimulation resulted in a visible increase in spleen cellularity and a certain increase in the number of cells with mature T-cell and macrophage surface markers. These results strongly suggest that immunostimulation increases the tolerance of ICR females to the teratogenic effect of STZ-induced diabetes. [Diabetologia (1997) 40: 635–640]