Using the microparticle capture enzyme-immunoassay (MEIA) based on IMx technology (Abbott), we determined the current prevalence of toxoplasmosis in 784 pregnant women followed up during 1990, and in 1,839 randomly selected blood donors. They all came from the Brabant Wallon area and the South-East of Brussels. Specimens yielding low IgG immunity (6-15 units) [corrected], were further tested with a sensitive direct agglutination assay (Toxo-Screen DA, bioMerieux). Overall, the prevalence was 67% among blood donors and 50% in pregnant women. In blood donors, the prevalence in women was not statistically different from the prevalence in men: X2 = 2.95 NS. In the two populations, a progressive age-related increasing prevalence of up to 60% for pregnant women and 77% for blood donors was observed. In females, the prevalence was higher among female blood donors than among pregnant women: 63% versus 50%, X2 = 16, P < 0.001. However, when the prevalences were compared within three age subgroups of women (< or = 33 yrs, 34 to 41 yrs, > or = 42 yrs), there were no statistically significant differences between pregnant women and blood donors. Thus, the overall observed difference was due to an age effect. Therefore, the distribution of IgG titers was established in each of the six age and sex subgroups. The 25th, 50th and 75th percentiles of those distributions ranged between 14 IU and 20 IU, 24 IU and 35 IU, and 40 IU and 64 IU, respectively. The annual seroconversion rate was 0.8% in pregnant women, against 0.2% amongst non-immune blood donors over 3 months. In conclusion, our findings confirm the general prevalence of 50% of toxoplasmosis and an annual seroconversion rate of 0.8% in these two populations.