Icteric hepatitis which developed in the first 6 months after blood transfusion was studied in 159 cases. One fourth of these (39) occurred within a period of only 15 days: between the 30th and 45th day after transfusion. So, their incubation period was longer than that of short incubation hepatitis (type A), and shorter than that of long incubation hepatitis (type B). In these cases the incidence of HBs antibodies was more than 10 times greater than in hepatitis with long incubation, and the incidence of HBs antigen was approximately the same as in the healthy control group. This fact suggests the involvement of a special factor in their appearance, probably an immunologic (or of other nature) process giving rise to antigen-antibody complexes in the HB ag-ab system.