Bovine IgG and albumin concentrations were determined from serum and semen of 59 bulls that were divided into 4 groups: 35 non-infected bulls (Group 1); 10 with vesiculitis due to Corynebacterium pyogenes (Group 2); 10 bulls with orchitis due to Chlamydia psittaci (Group 3); and 4 bulls with infectious vesiculitis (Group 4) sampled both before and after antibiotic treatment. Serum IgG concentrations (25 mg/ml approximately) were similar in non-infected (Gp 1) and infected bulls (Gp 2,3,4) whereas serum albumin concentrations were greater in infected than in non-infected bulls (51 mg/ml vs.41 mg/ml; p less than 0.01). By contrast, both semen IgG and albumin concentrations in infected bulls (0.47 and 0.54 mg/ml respectively) were significantly different from those of non-infected bulls (0.14 and 0.32 mg/ml; p less than 0.01). In addition, bulls with chlamydial orchitis had both semen (but not serum) IgG and albumin levels higher than those suffering from vesiculitis (p less than 0.01). Antibiotic therapy led to recovery and simultaneously to decreased concentrations of semen IgG and albumin. These results strongly suggest a local IgG synthesis or selective diffusion after such genital infections and further indicate that semen IgG and albumin assays could be a new and valuable tool for diagnosis and evaluation of genital infections.