Lymphoreticular cell (LRC) infiltration into tumour masses at different stages of growth after transplantation was studied in canine transmissible venereal sarcoma (CTVS). The percent viable LRC of total viable cells in tumour suspensions shows an inverse correlation with the logs of tumour mass of adult dogs (R = -0.475, phi = 28, P less than 0.01), which suggests that the degree of LRC infiltration is a measure of the host immune response. However, an estimate of the surface area (S.A.) of the tumour rather well perfused with blood elements based on the formula S.A. = 5.43 (tumour mass in g)2/3 showed that the LRC mass present in a tumour was a function of its surface area. Thus, the inverse correlation of the percent viable LRC's in a viable tumour cell suspension to the log of the CTVS mass might not necessarily indicate that the infiltrated LRC's were active as immune effector cells. The correlation does show that, as the CTVS size increases, the volume of the necrotic center increases more rapidly than the volume perfused with blood and, therefore, the percent viable LRC's in CTVS decreases with CTVS size increase.