Regression analysis was carried out in order to predict the effective concentration (EC₅₀) of 34 chemicals for inhibition of the growth of Tetrahymena pyriformis in 24 h. We examined 4 items (n-octanol/water partition coefficient (P), molecular weight (M_r), organic and inorganic characters, and molecular connectivity indices) and their derivatives. The best one-parameter equation was that involing M_r [log EC₅₀=-1.964 (M_r/100)+5.380 (r=-0.821, s=0.603, n=34)], and the second best involved the 3rd order valence molecular connectivity index of path type, ³χ^v_p[log EC₅₀=1.108 ³χ^v_p+3.944(r=-0.810, s=0.619, n=34)], while the n-octanol/water partition coefficient was least effective for the prediction of EC₅₀ values [log EC₅₀=-0.735 log P+4.233 (r=-0.780, s=0.660, n=34)]. The best combination for a two-parameter equation was ³χ^v_p and M_r [log EC₅₀=-1.105 (M_r/100)-0.638 ³χ^v_p+4.990 (R=0.866, s=0.516, n=34)]. Regression analysis with the same 4 items on median lethal concentration (LC₅₀) of 123 chemicals for Oryzias latipes (red killifish) was refered and the limitations in the application of the parameters and the equations botained are discussed.