The objective of this study was to estimate genetic parameters and environmental effects on somatic cell count in Croatian dairy cows. Data consisted of 861,417 test-day records for Simmental and 656,272 for Holstein cows. For Simmental breed, number of animals in pedigree was 123,199, while pedigree file for Holstein breed included 94,294 animals. A single-trait repeatability fixed regression test-day model was used to estimate genetic parameters. Fixed effects in the model were parity and calving season. Days in milk was fitted using Ali-Schaeffer lactation curve nested within parity. Age at first calving was modelled as quadratic regression. Direct additive genetic effect, herd, herd-test-date, and permanent environmental effect of cow within parity were included in the model as random effects. Variance components were estimated using Residual Maximum Likelihood method as implemented in the VCE-6 program. Estimated heritabilities were 0.21 for Simmental and 0.15 for the Holstein breed. Permanent environmental effect explained 19 % of phenotypic variation in Simmental and 20 % in Holstein breed. Herd and herd-test-date accounted for another 9 % and 5 % of variability for Simmental breed. The effects of herd and herd-test-date explained 10 % and 5 % of phenotypic variance in Holstein breed.Cilj ovog rada bio je ispitati i ocijeniti utjecaj okolišnih čimbenika na varijabilnost broja somatskih stanica u mlijeku i procijeniti komponente varijance. Ukupno je analizirano 861.417 dnevnih zapisa krava simentalske i 656.272 dnevna zapisa krava holstein pasmine. Porijeklo simentalske pasmine uključuje 123.999, a porijeklo holstein pasmine 94.294 životinje. Za procjenu genetskih parametara korišten je model dnevnih zapisa s fiksnom regresijom. Fiksni dio modela je uključivao utjecaj rednog broja laktacije i sezone teljenja. Stadij laktacije je opisan Ali-Schaefferovom funkcijom ugniježđenom unutar rednog broja laktacije, dok je dob kod prvog teljenja opisana kvadratnom regresijom. Slučajni dio modela je određen stadom, interakcijom stado-dan testiranja, direktnim aditivnim genetskim i stalnim okolišnim utjecajem. Procijenjeni heritabilitet za broj somatskih stanica u mlijeku krava iznosio je h2= 0,21 za simentalsku pasminu, a za holstein pasminu h2=0,15. Stalnim utjecajem okoliša je pojašnjeno 19 % varijabilnosti za simentalsku, te 20 % za holstein pasminu. Utjecajem stada i interakcije stado-dan kontrole za simentalsku pasminu je pojašnjeno 9 % odnosno 5 % od ukupne varijabilnosti broja somatskih stanica. Za holstein pasminu je utjecaj stada i interakcije stado-dan kontrole pojasnio 10 % odnosno 5 % varijabilnosti
