In this study, we examined the effect of antisense neu recombinant murine retroviral vectors on the p185 expression in B104-1 cells. Two fragments containing the 5' end and the transmembrane region of neu* were inserted in an inverted orientation relative to the 5' long terminal repeat (LTR) of the pDOL retroviral vector and used in transfecting B104-1 cells. The results obtained from RNAse protection assays were not consistent with the proposed mechanism of the antisense action by other investigators. Elevated expression of p185 was observed in several antisense vector-transfected clones, presumably caused by the promoter-insertion type of activation.