Fluid-phase immunoprecipitation analysis has undergone two distinct paths of development. One path utilizes existing absorptiometric equipment while the other relies on apparatus constructed specifically for the measurement of scattered light. The principles and instrumentation employed in light scattering and absorption techniques are reviewed with special reference to their applications in immunology. The main features of available analysers are summarised in the table. Specialised equipment is becoming increasingly sophisticated and expensive and it is important to assess critically the benefits, if any, that such apparatus may offer compared with more conventional devices. The latter are often capable of performing the same function with greater versatility. A good quality bench spectrophotometer is a more appropriate multi-purpose laboratory instrument than a specialised nephelometer with its more limited application. The equipment of choice for automated immunochemical protein quantitation, however, is the centrifugal analyser. The capital outlay is compensated for by high throughput, development potential and reduced consumption of reagents (i.e. antisera).