Preincubation of feline peripheral blood lymphocytes (PBL) in complete medium at 37 degrees C for up to 24 h resulted in as much as a 5.5-fold increase in the per cent of T cells detectable by spontaneous E rosette formation with guinea pig erythrocytes, as compared to non-preincubated PBL. Under these same conditions, the percent of B cells, detected by EAC rosette formation, remained unchanged. Preincubation with either thymosin or levamisole caused no increase in E rosettes other than that seen with complete medium alone. The preincubation-associated increase in E rosettes was temperature dependent, independent of the source of serum in the preincubation medium, not influenced by the lymphocyte isolation procedure, and not observed when feline thymocytes were tested in place of PBL. No increase in functional activity, assayed by phytomitogen-induced lymphocyte blastogenesis, was observed in preincubated as compared to non-preincubated PBL. These results suggested either a non-specific unmasking, or the differentiation and acquisition, of E receptors by feline PBL during the preincubation procedure.