We have characterized monospecific sheep antisera to human IgG, IgA and IgM using Ouchterlony double diffusion, immunoelectrophoresis, quantitative precipitation and nephelometric activity. The antisera were conjugated at high and low F/P ratios, absorbed and subsequently evaluated in both direct and indirect immunofluorescence. Antisera which exhibited excellent reactivity in double diffusion, IEP and quantitative precipitation but gave a poor nephelometric response (Ab dilution >1/25 gave no measurable response in the Technicon Fluoronephelometer) showed a marked decreased effectiveness in both direct and indirect fluorescence. Antisera which exhibited a strong nephelometric response were invariably excellent in double diffusion, IEP and quantitative precipitation and gave strong positive fluorescence at reasonable dilutions in both direct and indirect fluorescence. We suggest nεphelometry prior to conjugation is the method of choice in screening antisera usable in immunofluorescence.