Treatment of cows with a subclinical course of mastitis due to staphylococci was carried out using biostimulation (Bio) and chemotherapy. There was administered Biomast, elaborated by the authors, and if Bio was not effective, Pathozone-Phizer was given once. Biostimulation was performed in 121 (82.3%) out of 147 cows; they belonged to two herds and their productiveness was 4680 1 and 5900 1 respectively. Staphylococcal infection was found in 332 lobes (56.9%). Control examinations were conducted after 21, 49, 76, and 101 days. Using Bio 40.9% of cows was cured (52.47% of lobes). Differences in the frequency of udder infections in the control group of cows were not significant (p < 0.05). After the combined treatment with Bio and Pathozone there was reached 74.7% of recoveries (89.2% of lobes). Altogether 83.5% of cows was cured. Treatment with Pathozone only resulted in 64.9% recovery in 21 cows (57 lobes). It was found that the effectiveness of Pathozone was higher at 16.5% in cows prior treated with Bio (P < 0.001). The two methods of treatment gave a cumulative effect, i.e. Bio increased the effectiveness of Pathozone. Bio-chemotherapy should be applied in herds with a low effectiveness of biostimulation.