Three stages can be distinguished in the evolution of HA titer during Sendai virus multiplication in chorioallantoic membrane (CAM) fragments in relation to the age of CAM and conditions of inoculation and maintenance. Ceruloplasmin has an inhibitory effect on virus multiplication, more marked in the case of stationarily maintained CAM fragments and dependent on the relationship between the moment of ceruloplasmin addition and of virus inoculation. Maximum inhibition is achieved by inoculation of Sendai virus (1 Ha u) after 4 1/2-hour preincubation with ceruloplasmin (5 x 10(-7)M). The experimental data plead for the formation of a virus-ceruloplasmin complex able to inhibit both penetration and release of virus synthesized in the cell.