The nature of the plasmacytomagenic activity of pristane (2,6,10,14-tetramethylpentadecane) is poorly defined. However, evidence for tumor promoting properties of pristane has recently come forward that includes direct cellular effects on B lymphocytes; i.e., the plasmacytoma precursor cell. Bly et al. (Cancer Biochem. Biophys. 11, 1990, 145-154) observed changed membrane fluidities in lymphocytes after administration of pristane in vivo. We measured steady-state fluorescence polarization using DPH (1,6-diphenyl-1,3,5-hexatriene) and APCL (1-acyl-2-[12-(9-anthryl)-11-trans-dodecenoyl]-sn-glycero-3- phosphocholine) as probes in DOPC (L-alpha-dioleoylphosphatidylcholin) model membranes and membrane fractions derived from plasmacytoma cells after incorporation of pristane in vitro. In a previous investigation, we verified the in vitro uptake of pristane into DOPC bilayers under the conditions employed here (Gawrisch and Janz, Biochim. Biophys. Acta 1070, 1991, 409-418). However, neither in DOPC bilayers nor in plasmacytoma membrane fractions could we detect changes in fluorescence polarization after in vitro incorporation of pristane within reasonable error limits. Therefore, we suggest that the observed alterations in membrane fluidity in lymphocytes from pristane-treated animals are the indirect result of the in vivo treatment but not a direct effect of pristane on membrane fluidity.