Plasmid vector sequences were retained (vector+), or removed (vector-) from hybrid major histocompatibility complex gene constructs prior to microinjection of fertilized ova for the production of transgenic mice. In transgenic mice containing integrated vector+ gene constructs, low levels of class II cell surface determinants were detected on splenocytes from only two out of six independent lines. Class II membrane determinants were not detectable on splenocytes from the remaining four vector+ transgenic lines. Expression of transgene products did not correlate with transgene copy number which ranged from 1-10 copies. Low levels of mRNA transcripts were detected in thymic mRNA from vector+ lines. In contrast, high levels of thymic and splenic mRNA transcripts were detected in offspring from all four vector- transgenic lines. Spleen cells from the vector- transgenic animals also expressed high levels of the hybrid major histocompatibility complex transgene products. These results implicate plasmid vector sequences in the inhibition of expression of the hybrid class II-class I major histocompatibility complex genes in transgenic mice. This putative inhibition of transgene expression presumably occurs at the level of gene transcription.