The crucial step in cellular immortalisation seems to be the activation of telomerase, the enzyme that synthesizes telomere repeat ending sequences. Since the telomerase activity has been detected in almost all types of cancer tissue it has been proposed as new reliable tumor marker. This study was conducted to evaluate the significance of appearance of circulating RNA for telomerase subunits hTR and hTERT in the plasma of cancer patients. Seven healthy volunteers, 25 primary breast cancer patients (stage I-III), 29 patients with advanced malignant melanoma (stage III-IV), and 4 patients with advanced thyroid cancer (stage III-IV) were included in the study. The total RNA was extracted from plasma samples, reverse transcribed to cDNAs, and specific cDNAs for hTR and hTERT were amplified by semi-nested PCR. In healthy volunteers, the control GAPDH was positive in all, hTR was positive in 3 cases, and hTERT was negative in all 7 tested cases. Among 25 breast cancer patients, hTR was positive in 23, and hTERT in 12 patients. Two cases, that were hTR and hTERT negative, were at the same time negative also for GAPDH. Of the 29 patients with malignant melanoma, 24 were positive for hTR, and 17 for hTERT. Again, the 5 patients that were negative for hTR and hTERT, were negative also for the control GAPDH. In all plasma samples from thyroid cancer patients, hTR and hTERT were positive. In conclusion, the RT-PCR followed by semi-nested PCR is a highly sensitive method for detection of circulating RNA in the plasma. Among the telomerase subunits, only hTERT could serve as an unspecific tumor marker in the plasma of cancer patients.