diff --git a/src/cellflow/preprocessing/_wknn.py b/src/cellflow/preprocessing/_wknn.py
index 222a9dcf..4cede888 100644
--- a/src/cellflow/preprocessing/_wknn.py
+++ b/src/cellflow/preprocessing/_wknn.py
@@ -134,15 +134,16 @@ def transfer_labels(
         )
 
     wknn = ref_adata.uns[wknn_key]
+    labels_onehot = pd.get_dummies(ref_adata.obs[label_key].astype(str))
+    labels_mat = sparse.csr_matrix((labels_onehot > 0).astype(int))
 
-    scores = pd.DataFrame(
-        wknn @ pd.get_dummies(ref_adata.obs[label_key]),
-        columns=pd.get_dummies(ref_adata.obs[label_key]).columns,
-        index=query_adata.obs_names,
-    )
+    scores = wknn @ labels_mat
+    label_indices = np.array(scores.argmax(1)).flatten()
+    max_scores = scores.max(1).toarray().flatten()
 
-    query_adata.obs[f"{label_key}_transfer"] = scores.idxmax(1)
-    query_adata.obs[f"{label_key}_transfer_score"] = scores.max(1)
+    query_adata.obs[f"{label_key}_transfer"] = labels_onehot.columns[label_indices]
+    query_adata.obs[f"{label_key}_transfer"] = query_adata.obs[f"{label_key}_transfer"].astype("category")
+    query_adata.obs[f"{label_key}_transfer_score"] = max_scores
 
     if copy:
         return query_adata