Acrylonitrile-butadiene-styrene (ABS) resins are prepared by the mass or mass-suspension process employing a tertiary-butyl peroxy initiator having a 10-hour half-life between about 50.degree. and 95.degree. C. wherein initial polymerization of up to about 10 weight percent solids is conducted under plug flow conditions. From about 5 to 10 percent conversion to about 40 percent conversion is polymerized with agitation. The remaining polymerization is done with or without agitation. Improved impact resistance is obtained.