In order to determine whether trophoblast (or gestational choriocarcinoma) expresses the HLA-DR antigen or not, it was analysed using human gestational choriocarcinoma cell lines (GCH-1, GCH-1(m) and TAK-N) by the Northern hybridization method. TYK-nu (human undifferentiated ovarian carcinoma cell line), M-14 (human malignant melanoma cell line), L-14 (human B lymphocyte cell line), and KKNS-1, KKNS-2 and KKNS-3 (three human endometrial carcinoma cell lines) were also examined. Messenger ribonucleic acids (mRNAs) were prepared from 2 X 10(7) cells in each cell line and hybridized by HLA-DR alpha-chain complementary deoxyribonucleic acid (cDNA) probe (pDR alpha-1) and beta-chain cDNA probe (DK-10). Northern hybridization analysis revealed that GCH-1 transcribed at least two kinds of alpha-chain (16s and 23s) and beta-chain mRNA (15s and 23s). TYK-nu transcribed alpha- and beta-chain mRNA, L-14 transcribed only alpha-chain mRNA, and TAK-N appeared to transcribe only a little alpha-chain mRNA. The HLA-DR molecules were not, however, expressed in the other cell lines (GCH-1(m), M-14, KKNS-1, -2 and -3). These results are compared with those obtained by immunocytochemical methods in our laboratory.