Although amino acid sequences of the alpha- and beta-subunits of human choriogonadotropin (hCG) are known, only limited information is available on the disease state hCG. We have examined the amino acid sequences of the alpha- and beta-subunits of hCG from choriocarcinoma BeWo cells. The amino acid sequences were derived from the nucleotide sequences of BeWo cDNA clones of hCG alpha- and beta-subunits and were found to be identical with those of the normal subunits. It appears that the differences between the normal and the choriocarcinoma alpha- and beta-subunits of hCG reside primarily in the carbohydrates rather than the amino acid sequences. It may be pointed out that although coding and non-coding regions of BeWo cDNA clones of CG alpha and CG beta had several base changes from the hCG alpha and hCG beta cDNAs, these changes did not result in the alteration of their amino acid sequences. The longest BeWo alpha and beta cDNAs were 719 and 878 base pairs (bp) in length and lacked only 16 and 7 bp from the transcription start sites respectively. BeWo CG alpha cDNA had two base changes in the non-coding regions, one insertion of C at position 39 and another substitution of T for A at position 651, the latter change deleted one HindIII polymorphous site. The BeWo CG beta cDNA also had two base substitutions, A for G at 131 in the non-coding region and T for C at 807 position in the coding region.