Between October 1996 and May 1997,94 horses which were suspected of being infected with strongyles were examined clinically, and samples of faeces were examined for strongyle eggs and cyathostome larvae (L4) and adults. Blood samples were monitored for total protein, albumin and β-globulins. In 28 of the horses (30 per cent) cyathostome L4 and adults were detected in the faeces, and were significantly associated with the horses' condition, the occurrence of diarrhoea, with lower concentrations of total protein and albumin, and with higher percentages of β-globulin. Thirty-four of the horses (36 per cent) were excreting strongyle eggs, but in these animals the associations were with high concentrations of total protein and albumin, and lower percentages of β-globulin. The results showed that during the winter, a horse in poor condition which has diarrhoea, an albumin concentration less than 20 g/litre, and a ratio of albumin:globulin less than 0.7 is very likely to be infected with L4 and adult stages of cyathostomes.