Myosin light chain kinase (MLCK) is essential for myometrial contractions induced by calcium-mobilizing agonists. From the gene of vertebrate smooth muscle/non-muscle MLCK there are at least four proteins expressed. We have found that both a > 200 and a 137 kDa MLCK are equally expressed in human non-pregnant (NP) and term pregnant (P) uterine smooth muscle and confirmed that 19 kDa telokin (TK) is only expressed in P myometrium. In addition, we have observed that a MLCK immunogen at ~60 kDa is only expressed in NP myometrium, suggesting that its expression is inhibited during normal pregnancy in a hormonally dependent manner. However, when we compared pregnant myometrium from patients delivered preterm (PT) (< 34 weeks gestation), but not in labour (NIL), with PT patients in labour (IL) we found that PT(IL) samples expressed the ~ 60 kDa MLCK immunogen and thus displayed a NP phenotype whereas PT(NIL) samples did not express the protein and retained a pregnant phenotype. We hypothesize that the novel ~ 60 kDa MLCK immunogen contributes to the aberrent contractility associated with preterm labour. Experimental Physiology (2001) 86.2, 313-318.