Channel catfish (Ictalurus punctatus Rafinesque) produce a 100-kDa serum protein referred to as phosphorylcholine-reactive protein (PRP). PRP appears homologous to human C-reactive protein (CRP) i.e. it is an acute phase pentraxin with calcium-dependent reactivity with phosphorylcholine. In this study a immunoassay was employed to quantify PRP levels in sera of pond-raised catfish collected at monthly intervals (January 1990 to January 1992) from ponds in Mississippi. Serum PRP was also measured in laboratory-maintained catfish exposed to physical, environmental, or chemical stressors and for catfish infected with the fungus Saprolegnia sp. For pond-raised catfish the average level of serum PRP was 2·42 ± 0·85 mg ml-1 for healthy animals and 1·74 ± 0·94 mg ml-1 for those infected with Saprolegnia sp. In general PRP levels were maximal in summer and minimal in winter, coincident with similar extremes in pond-water temperature. Levels of serum PRP for laboratory-maintained catfish were higher and more variable (3·8 ± 2·4 mg ml-1) than those for pond-raised fish. Within 2-4 days after injection with turpentine, serum PRP levels in laboratory-maintained catfish rose to concentrations as high as 10·6mg ml-1, representing an 18-fold increase over pre-injection levels. In contrast, fish injected with pneumococcal C-polysaccharide or Saprolegnia sp. fungal homogenate exhibited transient decreases in serum PRP levels. Concentrations of serum PRP for catfish subjected to low temperature-shock decreased 14% compared to pre-shock values by day 6 and 86% by day 18. In contrast, by day 6 fish subjected to low temperature-shock and subsequently infected with Saprolegnia sp. exhibited PRP levels 57% lower than healthy fish and 50% lower than healthy, low temperature-shocked fish. Several lines of evidence suggest that reduced levels of serum PRP induced by low temperatures may play a major role(s) in determining the increased susceptibility of channel catfish to infection by Saprolegnia sp. during the winter months. © 1994 Academic Press Limited.