By a rosette assay with ox red blood cells (ORBC) sensitised with rabbit IgM and IgG antibodies (EA mu and EA gamma), receptors for IgM (Fc mu-R) and IgG (Fc gamma-R) on the surface of guinea pig lymphocytes in the different lymphoid organs were demonstrated. A double rosette assay with rabbit erythrocytes (RRBC) and EA mu or EA gamma for detection of T cells with Fc mu-R (T mu) or Fc gamma-R (T gamma) was also developed. Fc mu-R was detected on 40.1%, 48.7%, 35.0% and less than 1% of peripheral blood, lymph node, spleen and thymus cells respectively. By a double rosette assay with RRBC and EA mu, approximately 30% of peripheral blood, lymph node and spleen T cells were identified as T mu cells. T gamma cells as revealed by a double rosette of RRBC and EA gamma were found to be 3% and 5% respectively of peripheral blood and lymph node T cells, whereas much higher numbers of T cells in the spleen (32.7) were identified as T gamma cells. This double rosette assay of T mu and T gamma cells in the peripheral blood of guinea pigs which had received prednisolone showed that the proportion of T mu cells decreased early after drug administration and that T gamma cells increased in number. Thus the methods provide a useful means of detecting T mu and T gamma cells in guinea pigs. This assay system for T mu and T gamma cells in guinea pigs should be useful for studies of their variation or functional significance in the immune response.