Ten aminoacyl transfer RNA's prepared from human malignant trophoblastic cells (BeWo line) were compared with the corresponding aminoacyl transfer RNA's from normal human chorionic tissue by cochromatography on a RPC-5 column. Phenylalanyl transfer RNA (Phe-tRNA) of BeWo cells had, in addition to the single species of Phe-tRNA found in normal chorionic tissues, an early eluting component. When Phe-tRNA from the chorion was exposed to mild acid, which selectively excises the Y base, it eluted in the same position as the early eluting Phe-tRNA of BeWo cells. Therefore, the BeWo Phe-tRNA is partially undermodified. Tyrosyl transfer RNA of BeWo cells exhibited a broad-based peak which eluted later than the normal and probably consists of two or more tyrosyl transfer RNA's. Seryl transfer RNA of BeWo cells showed two peaks of acceptor activity, while seryl transfer RNA of normal chorion had a third peak that eluted at a higher salt concentration. In addition, in an early eluting methionyl and lysyl transfer RNA and in a late eluting arginyl transfer RNA from BeWo cells and normal charion, quantitative alterations were detected. The remaining four transfer RNA's, leucyl, aspartyl, valyl, and histidyl, from the two sources did not show any significant differences in elution profiles. These alterations of the chromatographic profile appeared to be due to new or altered species of transfer RNA. They were not due to differences in the aminoacyl transfer RNA synthetase. The transfer RNA methyltransferase capacity of the enzymes from BeWo cells was 2-fold higher than that of the enzymes extracted from the chorion.