Certain sub-lines of the murine B cell lymphoma BCL1 can be maintained in vitro and respond to cytokines including IL-2 and IL-5. BCL1 cells, as well as other B lymphomas, are difficult to synchronize using conventional techniques such as thymidine block or DNA synthesis inhibition. We have found that BCL1 cells maintained in Dulbecco's minimum essential medium (DMEM) with non-essential amino acids (NEAA) can be readily synchronized by culture in DMEM lacking NEAA. Within 10-18 h of medium replacement, 98% of BCL1 cells are 2 N in DNA content, suggesting that these cells are arrested in G0/G1. This population of BCL1 cells is viable and can be stimulated to enter S phase by culture in media containing NEAA; however, arrested cells did not appear to return synchronously into the cell cycle on addition of NEAA. A transient increase in levels of c-fos and c-myc mRNA was not detected after arrested BCL1 cells were stimulated to enter S phase, suggesting that arrested cells are in the G1 phase of the cell cycle, rather than G0. This technique for obtaining G1 arrested B lymphoma cells may prove useful in the analysis of molecular events that occur in B cells as a function of cell cycle position.