The high density lipoprotein (HDL) fraction involved in the precipitation with Staphylococcus capitis can be quantitated by single radial diffusion in agar containing an extract of the bacteria. Both whole serum and purified HDL produce a precipitation ring with bacterial extract. When dilutions of serum are tested, the squared radius of the precipitation ring plotted against serum concentration gives a linear relationship. Cumulative density flotation of HDL, followed by quantitative analysis of precipitating activity and apo A, indicates that the precipitating activity is present in the less dense fraction of serum HDL (i.e. HDL2). This suggestion is supported by the finding that the precipitating activity correlates positively with HDL2 (but not with HDL3) in human sera. Precipitating activity was higher in sera from women than from men, in accordance with the sex difference in serum HDL. The results raise the question whether precipitation with an extract of S. capitis might serve as a simple, direct method to assess serum HDL2 levels.