Summary.Antibodies to the HPA‐la antigen can elicit a condition in the new‐born known as neonatal alloimmune thrombocytopenia (NAITP). Retrospective and prospective studies have shown that there is a strong correlation between the presence of HLA‐DR3, HLA‐DRw52 in the mother and the antibody response to HPA‐la. HLA Class II molecules play an important role in the initiation of the immune response and it has been postulated that HPA‐la antibody production could be determined by the presence of a specific HLA Class II molecule at the surface of the antigen‐presenting cell. Thirty‐one HPA‐la negative women with HPA‐la antibodies (responders) and nine HPA‐la negative women without HPA‐la antibodies (non‐responders) were recruited. They were studied using serological HLA Class I and Class II typing and RFLP analysis with a DRβ probe. We found that all responders had the HLA‐DRw52a sub‐specificity confirming recently published data. Moreover, two of the nine non‐responders were also found to be HLA‐DRw52a. These results suggest that the HLA‐DRw52a molecule is necessary for HPA‐la antibody responsiveness but not sufficient. The results also indicate that in HPA‐la negative women the absence of HLA‐DRw52a is associated with a very low risk of being antibody producers and hence, is associated with a