Serum proteins were evaluated by agarose electrophoresis at periodic intervals between postinoculation days (PID) 2 and 35 in 4 litters of pigs inoculated orally at 36 to 38 hours of age with 70,000 sporulated Isospora suis oocysts and in 4 litters of age-matched noninoculated control pigs. In inoculated pigs, clinical disease characterized by vomiting and diarrhea began at PID 3 to 4 and was resolved by PID 11. Isospora suis oocysts were detected in feces of inoculated pigs from PID 5 to 25, with peak numbers present on PID 5 to 6. Of 43 pigs inoculated with I suis oocysts, 4(9.3%) died of coccidiosis, and samples were not obtained for serum protein evaluation. Of 39 noninoculated control pigs, all remained clinically normal, and I suis oocysts were not detected in their feces. Serum protein fraction values in inoculated and control groups compared at each sample collection time did not differ significantly, except at PID 15, when beta-globulin values were lower in inoculated pigs (P = 0.02). At PID 35, total serum proteins, albumin, and alpha 1-, beta-, and gamma-globulin values of inoculated pigs were lower than those of controls, but there were too few pigs examined for definitive statistical analysis. Differences in trends over time were observed between inoculated and control groups for several serum protein fractions. In inoculated pigs, total proteins (P less than 0.1) and beta-globulins (P less than 0.01) decreased with time, whereas those of control pigs increased. Similar differences in trends were noticed for albumin and alpha 1-globulins, but these were not significant.(ABSTRACT TRUNCATED AT 250 WORDS)