Somatic cell count (SCC) is the most widely used mastitis indicator. However, immunoglobulin G (IgG), part of the specific immune system, is transferred from blood to milk, and the extent of this transfer appears to be pathogen specific. As IgG measurement is not available for farmers, milk lactate dehydrogenase (LDH) activity has been considered a suitable mastitis indicator and a marker for the presence of IgG in milk. This study aims to analyze the correlation and linear regression of the variables SCC, LDH and IgG in mastitis produced by different bacteria, and to determine if LDH can be used as an indicator for elevated IgG. Four quarters of 38 cows on 2 dairy farms with automatic milking systems were sampled. Selection criteria was based on the composite milk SCC (>100,000 cell/mL) from each cow. Samples were measured for SCC, LDH, IgG and cultured for bacteriology. SCC, LDH and IgG data were analyzed using the CORR and GLM procedures of SAS. When milk samples were not grouped by bacterial population, IgG and LDH were the highest correlated variables (r = 0.49) compared with IgG and SCC (r = 0.41) or LDH and SCC (r = 0.41). Prediction equations for all variables had an R-square ≤0.24. After quarter milk samples were classified by bacteria (control, S. aureus, C. bovis, E. coli, other Staphylococcus spp. and S. uberis), different correlations and regression patterns were observed. Control and infected quarters showed a positive correlation between LDH and SCC, with the exception of those infected with E. coli and S. uberis. A positive correlation between SCC and IgG was observed in quarters where C. bovis was identified. LDH/SCC regression slopes differed from the control group. Several differences between LDH/SCC and LDH/IgG regression slopes were observed among bacterial infections. In conclusion, LDH appears to be a good indicator for some bacterial infections (namely C. bovis); however the use of both SCC and LDH may increase the mastitis detection rate, particularly infections caused by C. bovis, other Staphylococcus spp. and S. aureus.