Milk samples from two cheese plants with overlapping milk supplies were collected monthly for one year in an attempt to measure seasonal variation in the ability of milk and whey to support lactic culture growth . Treatments to control and minimize variability of milk or whey were evaluated to optimize stability in starter culture performance. Raw milk samples were tested for somatic cell counts, activity tests (modified Horrall- Elliker), acid degree values, and total plate counts. Activity (modified Horraii-EIIiker) and inhibitory tests were also performed on pasteurized, pasteurized-vacuumized and high heat milk treatments. Rennet whey (heated and unheated) was collected from raw and pasteurized-vacuumized milk and tested for lactic culture performance by monitoring growth under pH control for 16 h and measuring milli-equivalents of neutralizer (NaOH) added. Lactic culture performance and stability in raw milk was poor in all seasons. Culture performance in high heat milk was poor, but demonstrated good repeatability. Pasteurized milk supported good lactic culture performance and stability. Pasteurized-vacuumized milk provided excellent lactic culture performance and stability throughout the year. Culture performance during December through March demonstrated the greatest variation. The cultures performed more uniformly during April through August. September was a transition month. Cultures demonstrated uniformity and optimum culture activity during October and November.