The growth pattern of L. pomona was studied with regard to culture variables which may influence growth. The variables studied were various mediums, modifications of Stuart"s medium, percentage of serum enrichment, nutritive value of different serums, pattern of culture pH and Eh, influence of medium pH, effect of age of inoculum, density of inoculums, and influence of temperature upon growth. Daily growth was measured by a Coleman 7 nephelometer. Stuart"s medium was found superior to either Schuffner"s, Korthof"s, or Chang"s medium, in terms of growth and production of an antigen free of debris, simplicity of compounding and storage. Based upon the nephelometric response and the dry weight increase, growth is most rapid during the first 72 to 96 hrs. Modifications of Stuart"s medium by the use of different kinds of water as well as buffer salts combinations did not appreciably change the growth-supporting nature of the medium. The use of different concentrations of rabbit serum enrichment had an effect on both early and total growth. Serum levels of 8, 10, and 12% produced greater cell crops early in the growth cycle and greater total growth than lesser serum levels. For routine liquid culture propagation, rabbit serum was superior to bovine, equine, porcine, and ovine serum. Little change in cultural pH and Eh was observed. A starting medium pH of 7.4 was found optimal for the growth of L. pomona. Growth was decreased with a pH of 6.7 and lower, and was progressively inhibited by alkaline pH values of above 7.4. There was no significant difference on the growth pattern when inoculums of 3 to 8 days of age were used; whereas there was an early diminution in the growth pattern when inoculums of 15 days of age or older were used. Density of inoculum influenced the growth pattern of L. pomona. Lag brought about by minute inoculums disappeared as the density of inoculum was increased and levels of total growth approached equality after an extended incubation period. The temperature of incubation had a pronounced effect on the maximal crop of leptospiral cells produced. The optimal temperature for total cell crops and maximum growth rate was between 28.5[degree] and 29.5[degree]C. Cultivation at lower temperatures (25[degree]C or less) resulted in slower growth, whereas higher temperatures (32-37[degree]C) resulted in faster initial growth, although such higher temperatures had a progressively detrimental influence on the process of cell division when held for extended periods of time.