The growth of tick cells in Leibovitz's L--15 medium supplemented with various concentrations of fetal bovine serum (FBS), tryptose phosphate broth (TPB), and tick egg extract (TEE) was evaluated using a protein assay. A continuous cell line from Rhipicephalus appendiculatus (RA 243) was compared with young lines of cells isolated from embryos of R. appendiculatus (RAE 25) and Rhipicephalus sanguineus (RSE 8). We found fetal bovine serum and tryptose phosphate broth both to be essential supplements. The addition of tick egg extract further stimulated growth. The yield of cellular protein in both young and continuous lines of tick cells increased as a function of the concentration of tryptose phosphate broth from 0 to 10%, and fetal bovine serum from 2.5 to 20%. The growth of the RA 243 line correlated negatively with the size of the inoculum and positively with the concentration of fetal bovine serum, as the greatest increase in cell protein was obtained when cells were seeded at a low density into a medium containing 20% fetal bovine serum. The addition of an extract prepared from eggs of R. sanguineus or Hyalomma excavatum improved yields of cultures and promoted cell growth at low population densities. The protein yield increased as a function of tick egg extract concentration, but 0.8% inhibited growth of the RA 243 line. The RA 243 line could be propagated in a medium supplemented with 10% tryptose phosphate broth, 5% fetal bovine serum, and 0.5% tick egg extract.