A novel procedure involving the sequential use of two different antisense constructs has been used to inhibit the synthesis of involucrin in a hybrid cell line formed by the fusion of a human cervical carcinoma cell with a normal human keratinocyte (ESH100P6). In this cell line, and other similar hybrids, malignancy, as measured by progressive growth in vivo, is suppressed; and it has been shown that the keratinocyte imposes its own programme of terminal differentiation on the non-malignant hybrid cell. In particular, involucrin, a precursor of one of the major components of the cornified envelope of mature keratinocytes, continues to be produced. When, however, malignant segregants arise in the hybrid cell population, the terminal differentiation programme of the keratinocyte is not expressed and involucrin ceases to be made. It seemed possible that if the synthesis of involucrin, a critical marker of keratinocyte terminal differentiation, could be completely inhibited, this differentiation programme might be disrupted, and the malignant phenotype might then reappear in the non-malignant hybrids. This question was investigated further in the present paper. Total, and specific, inhibition of involucrin synthesis was indeed achieved by a sequential two-step antisense procedure, which might provide a systematic general method for the complete inactivation of other selected target genes.