Although the pathophysiological mechanisms leading to recurrent spontaneous abortion are still not fully understood, treatment schemes based on immunological principles have been advocated in recent years claiming that the production of the so-called blocking factor is being specifically stimulated. We investigated, retrospectively, whether active immunization can affect the production of immunoglobulin (Ig)G and IgM anticardiolipin antibodies. In a group of untreated recurrent spontaneous abortion patients (n = 9), the range of variation of cardiolipin antibodies, during consecutive controls taken at the same time interval as after immunization, was not statistically significant. In contrast to this, significant increases of both IgG and IgM antibodies occurred after active immunization with paternal leucocytes in 10 of 15, and in 6 of 15 cases, respectively. The mean basal and posttransfusion levels were: 7.26 +/- 2.53 and 30.15 +/- 23 U/mL for IgG and 2.26 +/- 1.2 and 6.82 +/- 5.6 U/mL for IgM, respectively. We conclude that active immunization with human lymphocytes leads to the production of antibodies against cardiolipin. This effect is exerted on both IgM and IgG antibodies.