Ten pregnant heifers were inoculated in both hind udder quarters with a mixture of Actinomyces pyogenes, Fusobacterium necrophorum and Peptostreptococcus indolicus. Development of the experimental mastitis was monitored by sequential clinical and bacteriological examinations, and by blood acute phase protein profiles. Sequential changes in plasma fibrinogen, serum haptoglobin, acid-soluble glyco-proteins and alpha 1-proteinase inhibitor activity were analysed and compared with both the clinical and bacteriological findings and the final outcome of the disease after calving. All ten heifers developed moderate to severe clinical mastitis. Four recovered completely, had a normal lactation after calving and exhibited only transient fever and moderate local signs. In six of the heifers the course of the disease was severe, and the inoculated quarters failed to produce milk after calving. The acute phase response of the four heifers that recovered was also significantly milder than that of the other six heifers. Haptoglobin and acid-soluble glycoproteins were most effective in indicating the severity of the infection and predicting the final outcome of the disease. Fibrinogen was a reliable indicator for detecting the presence of bacterial infection in all heifers. alpha 1-Proteinase inhibitor activity was of low diagnostic value in this study.