Summary The expression of endogenous retroviral genes in mammals may be etiologically related to genetic diseases including cancer. Recently, A-type human endogenous retroviral genomes have been cloned using the reverse transcriptase ( pol) genes of rodent intracisternal A-particles (IAP). In this report, RNA from human colon adenocarcinoma and surrounding mucosa was hybridized to mouse IAP pol and gag genes to examine the expression of human endogenous A-type retroviruses. Abundant, heterogeneous size, polyadenylated transcripts homologous to the mouse IAP pol gene were detected in both tissues. Transcripts homologous to the mouse IAP gag region were not found.