A Spiral Plater and a Microtiter system were used to isolate and evaluate cultures for a paired strain culture program. Bacteriophage and temperature sensitivity data of 43 Streptococcus cremoris strains were introduced into a computer cluster program to pair the least similar strains. Selected pairs were challenged with phage. Resistant mutants were developed. Characteristics of proteinase positive and proteinase negative variants were examined. Proteinase positive isolates produced more changes in pH, cell mass and more generations in milk than their counter-parts. Paired proteinase negative cultures produced more change in pH and cell mass and more generations in milk than single strains. Whey based medium under pH control was superior to commercial internal pH control medium for proteinase negative culture propagation. Proteinase negative isolates achieved 90% of the cell mass obtained by their counterparts in nonfat dry milk-yeast medium. Proteinase negative starter culture endured significantly higher phage titers than proteinase positive cells. Proteinase negative variants sustained activity comparable with phage-free controls when challenged for seven cycles with high phage titers. Proteinase positive cells had impaired activity after the second cycle. Pairing of proteinase positive strains was advantageous for phage protection. Erythromycin, streptomycin and penicillin adversely affected the activity of both cell types, yet proteinase positive cells were significantly more inhibited. Pairing neither variant enhanced activity. Cheddar cheese was exclusively manufactured with 2% inoculum of proteinase negative cultures compared to 1.5% usage of the proteinase positive paired strains. Cheese quality and cheese making times were normal. Over 4200 consecutive vats of Cheddar cheese were made in 1982 employing one pair of proteinase positive culture. Acid control and cheese quality were improved. The cheese making times were more uniform. Smaller inocula volumes could successfully be used for bulk starter in cheese plants utilizing pH controlled starter propagation. A needle/syringe system for inoculating starter tanks provided better protection against contamination during inoculation.