Neutron activation was used by Johnson and Stross to determine the manganese content of the clay used in 11 sherds from Cuicuilco and Teotihuacán in the Valley of Mexico. The results indicate that three distinctive sherds of negative ware from Cuicuilco differ from local Late Preclassic wares but are similar to pottery of the Tzacualli phase made locally at Teotihuacán. Trade between the Chimalhuacan settlement at Cuicuilco and the Tzacualli town at Teotihuacán is thus indicated.