We studied the distribution of the basement membrane components laminin and type IV collagen in 46 serous tumors of the ovary, including a group of low malignant potential tumors with microinvasion. The findings were correlated with the expression of the 72 kDa type IV collagenase, an enzyme that initiates the degradation of type IV collagen and consequently may play a role in the process of invasion. Benign cystadenomas and tumors of low malignant potential without microinvasion showed a continuous basement membrane; whereas invasive carcinomas, peritoneal implants, and lymph node metastasis had frequent disruptions and extensive areas without basement membrane components. Early invasion in tumors of low malignant potential was characterized by focal disruptions in basement membranes and complete absence of laminin and type IV collagen around single or clusters of microinvasive cells. Type IV collagenase was negative or minimally expressed in cystadenomas, whereas in invasive carcinomas and metastasis the reactivity was moderate to intense. Microinvasive cells in tumors of low malignant potential were strongly positive. The collagenase IV was also localized in cell clusters elsewhere in the tumors where the basement membrane was still preserved. These cells had a similar morphology to that of the microinvasive cells. We conclude that detection of basement membrane components may be useful in recognizing early invasion in this group of ovarian neoplasms. The correlation between progressive anomalies of the basement membrane and expression of type IV collagenase suggests that this enzyme functions directly in the degradation of basement membrane components and facilitates the invasive process.