Mongolian gerbils (Meriones unguiculatus) are widely used as animal models for a variety of infectious diseases. However, immunological reagents such as cytokines have not been characterized. Two heterohybridomas, D9(E6)C2B3 and D9(E4), obtained by fusion of gerbil splenocytes with mouse myeloma cells (P3-X63-Ag8.653), expressed gerbil CD3G mRNA. These cells were suggested to be T cell heterohybridomas. They also expressed gerbil IL6 [D9(E6)C2B3] and TGFB [D9(E4) and D9(E6)C2B3] mRNAs. The addition of conditioned medium (CM) obtained from the culture of D9(E6)C2B3 significantly enhanced antibody secretion and expression of gerbil Cγ1 and Cε IGHC mRNAs in the B11D2(C2) heterohybridoma, which secretes gerbil IgG1. However, the addition of CM from both heterohybridomas did not improve in proliferation of B11D2(C2) cells. These results indicate that CM from D9(E6)C2B3 improved the culture of gerbil–mouse heterohybridomas, possibly by secreting gerbil IL6.