Transcription of the c-fos gene is known to be induced transiently by many types of cellular stimuli in various cultured cell lines; however, several authors have reported that the c-fos gene is constitutively transcribed in lymphoid cells. We detected, in fact, abundant transcripts which hybridized with a v-fos DNA probe in nuclear run-off transcripts and poly(A)+ RNA of both quiescent mouse splenic lymphocytes and unstimulated monocytic tumor cell lines. However, human c-fos cDNA did not hybridize with most of these transcripts. No signal was detected by v-fos probe in nuclear run-off transcripts of 3T3 fibroblasts, and c-fos was inducible in both the 3T3 cells and the monocytic tumor cell lines. In quiescent lymphocytes, only the 0.3 kb HincII-PvuII portion of v-fos DNA, which contains a repeat of CAAAA, hybridized with these transcripts; neither other parts of v-fos nor human c-fos DNAs did. These results suggest that a significant portion of the previously reported 'constitutive' transcripts detected by v-fos DNA in lymphocytes and monocytes are not transcripts of c-fos but of other sequences which are specifically expressed in lymphoid cells and have homology with the 0.3 kb HincII-PvuII fragment of v-fos.