For providing biocides such as 10,10ʹ-oxybisphenoxarsive (OBPA) in concentrated, immobilized form to an end use resin composition containing a primary thermoplastic resin with which the primary resin is incompatible at high concentrations, solid biocide resin concentrates are provided which immobilize high concentrations of biocide. The solid biocide resin concentrates may contain an alloy of two thermoplastic resins, one of which is substantially identical to the primary resin and one of which enables incorporation and immobilization of high concentrates of biocide in the solid biocide resin concentrates. The solid biocide resin concentrate may contain, in whole or in part, a thermoplastic copolymer having mer units such as are present in the primary resin in addition to dissimilar mer units which enhance compatibility of the biocide with the copolymer.