Abstract: Equations of quantitative structure-activity relationships for Ieucomycins, lincomycins and clindamycins have been established by the method of molecular connectivity. The in vitro antibacterial activity of derivatives of the leucomycin complex increases with increasing the firstorder term of valence molecular connectivity (¹χ^v) and removal of esterification of the 3-hydroxy of the macrolide ring. In the series of NCH₃ and NC₂H₅ lincomycins, an optimum ¹χ^vO of 11.7583 and of 12.2394 was calculated respectively for in vitro activity vs Sarcine lutea An optimum ¹χ^vO of 11.3604 was calculated too. It was compared with Hansch method for these three series of derivatives set by Martin. Two methods give almost the same results. Molecular connectivity method based on calculation is found to have a more significant correlation in statistics than Hansch method which depends on empiric parameters.