Antimicrobial guanidine compounds are used, either alone or with at least one other antimicrobial compound, to make mildew-resistant wallboard. Particularly useful co-additives are o-phenyl phenol and sodium methyldithiocarbamate. The antimicrobial compound(s) may be added to the covering, the core body, an adhesive at the interface of the core body and the covering, or all three of these sites. A preferred guanidine compound is dodecyl guanidine hydrochloride.