AbstractCalmodulin is encoded in vertebrates by three different genes: CALM1, CALM2, and CALM3. We have examined the mRNAs expressed from these three genes in eight lines of human lymphoblastoid cells (Namalwa, Raji, Ramos, JY, Molt‐4, Jurkat, CEM, and HPB‐ALL). We found that all these cell lines (except Ramos) overexpressed CALM3 transcripts, which led to an increase of total CaM protein with respect to quiescent normal I lymphocytes. The nuclear concentration of calmodulin was measured in two of these lymphoblastoid cell lines (JY and HPB‐ALL) and compared to quiescent and phytohemagglutinin‐activated T lymphocytes. Activated lymphocytes showed a 2‐fold increase of nuclear calmodulin with respect to quiescent cells, whereas in the two lymphoblastoid cell lines, nuclear calmodulin remained similar to that of quiescent cells. The levels of a calmodulin‐binding protein of 150 kDa in the homogenates of the eight lymphoblastoid lines was found to be higher than those of quiescent and activated lymphocytes. Likewise, the amount of three calmodulin‐binding proteins of 240, 200, and 170 kDa was also increased in several of the cell lines, but not in all of them. The 170‐kDa protein was only expressed by activated lymphocytes and lymphoblastoid cells, suggesting that it could be specific for proliferating cells. In the nuclei of activated lymphocytes and lymphoblastoid cells, a decrease of a calmodulin‐binding protein of 110 kDa and increases of three other of 240, 180 and 170 kDa were also detected. © 1994 wiley‐Liss, Inc.