The present study was undertaken to compare the technetium-99m labelled non-specific polyclonal human immunoglobulin (Ig) with 99mTc-labelled monomeric human immunoglobulin (m-Ig), 99mTc-labelled, protein A-purified, human immunoglobulin (A-Ig) and 99mTc-labelled monomeric, protein A-purified, human immunoglobulin (mA-Ig) as tracer agents for the detection of a thigh infection with Staphylococcus aureus. In vitro the binding of the various tracer agents to bacteria at various intervals was determined. For the in vivo evaluation, mice were infected and received one of the various labelled proteins. Scintigrams were made 0.25, 1, 4 and 24 h later. All 99mTc-labelled Igs bound to bacteria in vitro: the percentages of binding for the m-Ig (from 1 h onwards) and A-Ig and mA-Ig (from 3 h onwards) were significantly higher than that for Ig. The in vivo target-to-non-target (T/NT) ratios were significantly higher from 4 h onwards for all purified Igs than for Ig. Protein A-purified Igs yielded higher T/NT ratios than m-Ig. Furthermore, the amount of activity in the liver was significantly lower 24 h after administration of m-Ig, A-Ig and mA-Ig than after administration of Ig. It is concluded that in this experimental infection 99mTc-labelled monomeric Ig localizes a staphylococcal thigh infection better and faster than 99mTc-labelled unpurified Ig. However, the accumulation obtained with protein A-purified Ig or protein A-purified monomeric Ig was the highest of all tracer agents tested.