Several strategies have been proposed for the research of human anti-Ia alloantisera. The present work evaluates the results of a screening made on multiparous sera, using either platelet-absorbed sera or unabsorbed sera against cells coated by anti-human-beta-2-microglobulin antibodies. When the results of the two approaches were compared, it was observed that the latter shows a higher frequency of positive reactions, being able to detect weak anti-Ia, and possibly non-HLA antibodies, which are not revealed with the usual test on platelet-absorbed serum. A significant correlation has been noted between the presence of anti-HLA-A,B,C and of anti-Ia, demonstrating that the best source of Ia antibodies from multiparous women are sera also containing anti-HLA-A,B,C.