In this study eighty five chicks were used. The chicks were infected with 2000 sporulated E.necatrix oocysts each and vaccinated with HB, vaccine when they were 7 days old. The chickens in the group (1) inoculated with E.necatrix oocysts 3 days before vaccination showed that: 1) The maximum number of oocysts out put (61100) was recorded on the 8th day post infection (5 days post vaccination). 2) There was a great difference between the Geometric mean titre (C.M.T) of the group (4.35) and that of control (vaccin ated non infected) which reached up to (6.94). The chickens in the group (2) were inocuiated with E.necatrix oocysts simultaneously with vaccination showed that: 1) Depressive effect of the 3rd generation schizont on the antibody formation. 2) The number of oocysts during the first 4 days after shedding of the oocy sts was larger than that of the control which reflect the stress effect oi vaccination.