Summary The production of interferon by human diploid cells stimulated by poly-I:C can be increased by pretreatment of the cells with interferon (priming effect). Large amounts of interferon (30000 units/ml) can be obtained by combining priming with a superinduction schedule adapted from that described for rabbit kidney cultures (Tan et al. 1970; Vilcek & Ng, 1971). Human plasma protein could replace bovine serum albumin in the production medium.