Mice with intraperitoneally inoculated leukemia L1210 cells had a lower total number of thymocytes and a markedly reduced percentage of L3T4+ Lyt2+ (CD4+ 8+) thymocytes, but increased subpopulations of L3T4+ Lyt2-, L3T4- Lyt2+, L3T4-Lyt2-. The percentage of Thy 1.2+ thymocytes was unchanged as compared with control animals. A similar effect was observed is suggested that leukemia L1210 cells produce some activities which diffuse through 0.22 microns filters and influence the subpopulations of thymocytes.